49742d4ecc
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@31308 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
1230 lines
55 KiB
TeX
1230 lines
55 KiB
TeX
\section{Database classes overview}\label{odbcoverview}
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Following is a detailed overview of how to use the wxWidgets ODBC classes - \helpref{wxDb}{wxdb}
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and \helpref{wxDbTable}{wxdbtable} and their associated functions. These are
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the ODBC classes donated by Remstar International, and are collectively
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referred to herein as the wxODBC classes.
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\subsection{wxDb/wxDbTable wxODBC Overview}\label{wxodbcoverview}
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Classes: \helpref{wxDb}{wxdb}, \helpref{wxDbTable}{wxdbtable}
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The wxODBC classes were designed for database independence. Although SQL and
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ODBC both have standards which define the minimum requirements they must
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support to be in compliance with specifications, different database vendors
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may implement things slightly differently. One example of this is that Oracle
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requires all user names for the datasources to be supplied in uppercase
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characters. In situations like this, the wxODBC classes have been written
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to make this transparent to the programmer when using functions that require
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database-specific syntax.
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Currently several major databases, along with other widely used databases,
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have been tested and supported through the wxODBC classes. The list of
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supported databases is certain to grow as more users start implementing
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software with these classes, but at the time of the writing of this document,
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users have successfully used the classes with the following datasources:
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\begin{itemize}\itemsep=0pt
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\item DB2
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\item DBase (IV, V)**
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\item Firebird
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\item INFORMIX
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\item Interbase
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\item MS SQL Server (v7 - minimal testing)
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\item MS Access (97, 2000, 2002, and 2003)
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\item MySQL (2.x and 3.5 - use the 2.5x drivers though)
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\item Oracle (v7, v8, v8i)
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\item Pervasive SQL
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\item PostgreSQL
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\item Sybase (ASA and ASE)
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\item XBase Sequiter
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\item VIRTUOSO
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\end{itemize}
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An up-to-date list can be obtained by looking in the comments of the function
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\helpref{wxDb::Dbms}{wxdbdbms} in db.cpp, or in the enumerated type
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\helpref{wxDBMS}{wxdbenumeratedtypes} in db.h.
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**dBase is not truly an ODBC datasource, but there are drivers which can
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emulate much of the functionality of an ODBC connection to a dBase table.
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See the \helpref{wxODBC Known Issues}{wxodbcknownissues} section of this
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overview for details.
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\subsection{wxODBC Where To Start}\label{wxodbcwheretostart}
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First, if you are not familiar with SQL and ODBC, go to your local bookstore
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and pick up a good book on each. This documentation is not meant to teach
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you many details about SQL or ODBC, though you may learn some just from
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immersion in the subject.
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If you have worked with non-SQL/ODBC datasources before, there are some
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things you will need to un-learn. First some terminology as these phrases will
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be used heavily in this section of the manual.
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\begin{twocollist}\itemsep=0pt
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\twocolitem{Datasource}{(usually a database) that contains the data that will be
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accessed by the wxODBC classes.}
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\twocolitem{Data table}{The section of the datasource that contains the rows and
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columns of data.}
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\twocolitem{ODBC driver}{The middle-ware software that interprets the ODBC
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commands sent by your application and converts them to the SQL format expected
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by the target datasource.}
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\twocolitem{Datasource connection}{An open pipe between your application and
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the ODBC driver which in turn has a connection to the target datasource.
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Datasource connections can have a virtually unlimited number of wxDbTable
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instances using the same connect (dependent on the ODBC driver). A separate
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connection is not needed for each table (the exception is for isolating
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commits/rollbacks on different tables from affecting more than the desired
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table. See the class documentation on
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\helpref{wxDb::CommitTrans}{wxdbcommittrans} and
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\helpref{wxDb::RollbackTrans}{wxdbrollbacktrans}.)}
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\twocolitem{Rows}{Similar to records in old relational databases, a row is a
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collection of one instance of each column of the data table that are all
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associated with each other.}
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\twocolitem{Columns}{Individual fields associated with each row of a data
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table.}
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\twocolitem{Query}{Request from the client to the datasource asking for
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the data that matches the requirements specified in the users request. When
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a query is performed, the datasource performs the lookup of the rows with
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satisfy the query, and creates a result set.}
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\twocolitem{Result set}{The data which matches the requirements specified
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in a query sent to the datasource. Dependent on drivers, a result set
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typically remains at the datasource (no data is transmitted to the ODBC driver)
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until the client actually instructs the ODBC driver to retrieve it.}
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\twocolitem{Cursor}{A logical pointer into the result set that a query
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generates, indicating the next record that will be returned to the client
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when a request for the next record is made.}
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\twocolitem{Scrolling cursors}{Scrolling refers to the movement of cursors
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through the result set. Cursors can always scroll forward sequentially in
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the result set (FORWARD ONLY scrolling cursors). With Forward only scrolling
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cursors, once a row in the result set has been returned to the ODBC driver
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and on to the client, there is no way to have the cursor move backward in
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the result set to look at the row that is previous to the current row in
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the result set. If BACKWARD scrolling cursors are supported by both the
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ODBC driver and the datasource that are being used, then backward
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scrolling cursor functions may be used (
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\helpref{wxDbTable::GetPrev}{wxdbtablegetprev},
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\helpref{wxDbTable::GetFirst}{wxdbtablegetfirst}, and
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\helpref{wxDbTable::GetLast}{wxdbtablegetlast}). If the datasource or the
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ODBC driver only support forward scrolling cursors, your program and logic
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must take this in to account.}
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\twocolitem{Commit/Rollback}{Commit will physically save
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insertions/deletions/updates, while rollback basically does an undo of
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everything done against the datasource connection that has not been
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previously committed. Note that Commit and Rollbacks are done on a
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connection, not on individual tables. All tables which use a shared
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connection to the datasource are all committed/rolled back at the same
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time when a call to
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\helpref{wxDb::CommitTrans}{wxdbcommittrans} or
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\helpref{wxDb::RollbackTrans}{wxdbrollbacktrans} is made.}
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\twocolitem{Index}{Indexes are datasource-maintained lookup structures
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that allow the datasource to quickly locate data rows based on the values
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of certain columns. Without indexes, the datasource would need to do a
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sequential search of a table every time a query request is made. Proper
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unique key index construction can make datasource queries nearly instantaneous.}
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\end{twocollist}
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Before you are able to read data from a data table in a datasource, you must
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have a connection to the datasource. Each datasource connection may be used
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to open multiple tables all on the same connection (number of tables open are
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dependent on the driver, datasource configuration and the amount of memory on
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the client workstation). Multiple connections can be opened to the same
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datasource by the same client (number of concurrent connections is dependent
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on the driver and datasource configuration).
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When a query is performed, the client passes the query to the ODBC driver,
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and the driver then translates it and passes it along to the datasource. The
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database engine (in most cases - exceptions are text and dBase files) running
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on the machine hosting the database does all the work of performing the search
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for the requested data. The client simply waits for a status to come back
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through the ODBC driver from the datasource.
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Depending on the ODBC driver, the result set either remains "queued" on the
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database server side, or is transferred to the machine that the driver is
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queued on. The client does not receive this data. The client must request
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some or all of the result set to be returned before any data rows are
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returned to the client application.
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Result sets do not need to include all columns of every row matching the
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query. In fact, result sets can actually be joinings of columns from two
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or more data tables, may have derived column values, or calculated values
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returned.
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For each result set, a cursor is maintained (typically by the database)
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which keeps track of where in the result set the user currently is.
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Depending on the database, ODBC driver, and how you configured the
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wxWidgets ODBC settings in setup.h (see \helpref{wxODBC - Compiling}{wxodbccompiling}), cursors can be
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either forward or backward scrolling. At a minimum, cursors must scroll
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forward. For example, if a query resulted in a result set with 100 rows,
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as the data is read by the client application, it will read row 1, then 2,
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then 3, etc. With forward only cursors, once the cursor has moved to
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the next row, the previous row cannot be accessed again without re-querying
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the datasource for the result set over again. Backward scrolling cursors
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allow you to request the previous row from the result set, actually
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scrolling the cursor backward.
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Backward scrolling cursors are not supported on all database/driver
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combinations. For this reason, forward-only cursors are the default in
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the wxODBC classes. If your datasource does support backward scrolling
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cursors and you wish to use them, make the appropriate changes in setup.h
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to enable them (see \helpref{wxODBC - Compiling}{wxodbccompiling}). For greatest portability between
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datasources, writing your program in such a way that it only requires
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forward scrolling cursors is your best bet. On the other hand, if you are
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focusing on using only datasources that support backward scrolling cursors,
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potentially large performance benefits can be gained from using them.
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There is a limit to the number of cursors that can be open on each connection
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to the datasource, and usually a maximum number of cursors for the datasource
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itself. This is all dependent on the database. Each connection that is
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opened (each instance of a wxDb) opens a minimum of 5 cursors on creation
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that are required for things such as updates/deletions/rollbacks/queries.
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Cursors are a limited resource, so use care in creating large numbers of
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cursors.
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Additional cursors can be created if necessary with the
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\helpref{wxDbTable::GetNewCursor}{wxdbtablegetnewcursor} function. One example
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use for additional cursors is to track multiple scroll points in result
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sets. By creating a new cursor, a program could request a second result set
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from the datasource while still maintaining the original cursor position in
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the first result set.
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Different than non-SQL/ODBC datasources, when a program performs an
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insertion, deletion, or update (or other SQL functions like altering
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tables, etc) through ODBC, the program must issue a "commit" to the
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datasource to tell the datasource that the action(s) it has been told to
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perform are to be recorded as permanent. Until a commit is performed,
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any other programs that query the datasource will not see the changes that
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have been made (although there are databases that can be configured to
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auto-commit). NOTE: With most datasources, until the commit is
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performed, any cursor that is open on that same datasource connection
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will be able to see the changes that are uncommitted. Check your
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database's documentation/configuration to verify this before relying on it
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though.
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A rollback is basically an UNDO command on the datasource connection. When
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a rollback is issued, the datasource will flush all commands it has been told
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to do since the last commit that was performed.
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NOTE: Commits/Rollbacks are done on datasource connections (wxDb instances)
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not on the wxDbTable instances. This means that if more than one table
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shares the same connection, and a commit or rollback is done on that
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connection, all pending changes for ALL tables using that connection are
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committed/rolled back.
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\subsection{wxODBC - Configuring your system for ODBC use}\label{wxodbcconfiguringyoursystem}
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Before you are able to access a datasource, you must have installed and
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configured an ODBC driver. Doing this is system specific, so it will not be
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covered in detail here. But here are a few details to get you started.
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Most database vendors provide at least a minimal ODBC driver with their
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database product. In practice, many of these drivers have proven to be slow
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and/or incomplete. Rumour has it that this is because the vendors do not want
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you using the ODBC interface to their products; they want you to use their
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applications to access the data.
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Whatever the reason, for database-intensive applications, you may want to
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consider using a third-party ODBC driver for your needs. One example of a
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third-party set of ODBC drivers that has been heavily tested and used is
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Rogue Wave's drivers. Rogue Wave has drivers available for many different
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platforms and databases.
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Under Microsoft Windows, install the ODBC driver you are planning to use. You
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will then use the ODBC Administrator in the Control Panel to configure an
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instance of the driver for your intended datasource. Note that with all
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flavors of NT, this configuration can be set up as a System or User DSN
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(datasource name). Configuring it as a system resource will make it
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available to all users (if you are logged in as 'administrator'), otherwise
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the datasource will only be available to the user who configured the DSN.
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Under Unix, iODBC is used for implementation of the ODBC API. To compile the
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wxODBC classes, you must first obtain iODBC from \urlref{http://www.iodbc.org}{www.iodbc.org} and install it.
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(Note: wxWidgets currently includes a version of iODBC.) Then you must create the file "~/.odbc.ini" (or optionally create
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"/etc/odbc.ini" for access for all users on the system). This file contains
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the settings for your system/datasource. Below is an example section of a
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odbc.ini file for use with the "samples/db" sample program using MySQL:
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\begin{verbatim}
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[contacts]
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Trace = Off
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TraceFile= stderr
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Driver = /usr/local/lib/libmyodbc.so
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DSN = contacts
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SERVER = 192.168.1.13
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USER = qet
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PASSWORD =
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PORT = 3306
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\end{verbatim}
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\subsection{wxODBC - Compiling}\label{wxodbccompiling}
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The wxWidgets setup.h file has several settings in it pertaining to compiling
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the wxODBC classes.
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\begin{twocollist}\itemsep=0pt
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\twocolitem{wxUSE\_ODBC}{This must be set to 1 in order for the compiler to
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compile the wxODBC classes. Without setting this to 1, there will be no
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access to any of the wxODBC classes. The default is 0.}
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\twocolitem{wxODBC\_FWD\_ONLY\_CURSORS}{When a new database connection is
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requested, this setting controls the default of whether the connection allows
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only forward scrolling cursors, or forward and backward scrolling cursors
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(see the section in "WHERE TO START" on cursors for more information on
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cursors). This default can be overridden by passing a second parameter to
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either the \helpref{wxDbGetConnection}{wxdbfunctions} or
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\helpref{wxDb constructor}{wxdbctor}. The default is 1.}
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\twocolitem{wxODBC\_BACKWARD\_COMPATABILITY}{Between v2.0 and 2.2, massive
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renaming efforts were done to the ODBC classes to get naming conventions
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similar to those used throughout wxWidgets, as well as to preface all wxODBC
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classes names and functions with a wxDb preface. Because this renaming would
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affect applications written using the v2.0 names, this compile-time directive
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was added to allow those programs written for v2.0 to still compile using the
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old naming conventions. These deprecated names are all {\tt\#}define'd to their
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corresponding new function names at the end of the db.cpp/dbtable.cpp source
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files. These deprecated class/function names should not be used in future
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development, as at some point in the future they will be removed. The default
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is 0.}
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\end{twocollist}
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{\it Under MS Windows}
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You are required to include the "odbc32.lib" provided by your compiler vendor
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in the list of external libraries to be linked in. If using the makefiles
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supplied with wxWidgets, this library should already be included for use with
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makefile.b32, makefile.vc, and makefile.g95.
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\normalbox{MORE TO COME}
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{\it Under Unix}
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--with-odbc flag for configure
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\normalbox{MORE TO COME}
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\subsection{wxODBC - Basic Step-By-Step Guide}\label{wxodbcstepbystep}
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To use the classes in an application, there are eight basic steps:
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\begin{itemize}\itemsep=0pt
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\item Define datasource connection information
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\item Get a datasource connection
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\item Create table definition
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\item Open the table
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\item Use the table
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\item Close the table
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\item Close the datasource connection
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\item Release the ODBC environment handle
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\end{itemize}
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Following each of these steps is detailed to explain the step, and to
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hopefully mention as many of the pitfalls that beginning users fall in
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to when first starting to use the classes. Throughout the steps, small
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snippets of code are provided to show the syntax of performing the step. A
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complete code snippet is provided at the end of this overview that shows a
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complete working flow of all these steps (see
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\helpref{wxODBC - Sample Code}{wxodbcsamplecode1}).
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{\bf Define datasource connection information}
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To be able to connect to a datasource through the ODBC driver, a program must
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supply a minimum of three pieces of information: Datasource name, User ID, and
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Authorization string (password). A fourth piece of information, a default
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directory indicating where the data file is stored, is required for Text and
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dBase drivers for ODBC.
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The wxWidgets data class wxDbConnectInf exists for holding all of these
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values, plus some others that may be desired.
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The 'Henv' member is the environment handle used to access memory for use by the
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ODBC driver. Use of this member is described below in the "Getting a Connection
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to the Datasource" section.
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The 'Dsn' must exactly match the datasource name used to configure the ODBC
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datasource (in the ODBC Administrator (MSW only) or in the .odbc.ini file).
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The 'Uid' is the User ID that is to be used to log in to the datasource. This
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User ID must already have been created and assigned rights within the
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datasource to which you are connecting. The user that the connection is
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establish by will determine what rights and privileges the datasource
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connection will allow the program to have when using the connection that
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this connection information was used to establish. Some datasources are
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case sensitive for User IDs, and though the wxODBC classes attempt to hide
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this from you by manipulating whatever data you pass in to match the
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datasource's needs, it is always best to pass the 'Uid' in the case that
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the datasource requires.
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The 'AuthStr' is the password for the User ID specified in the 'Uid' member.
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As with the 'Uid', some datasources are case sensitive (in fact most are).
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The wxODBC classes do NOT try to manage the case of the 'AuthStr' at all.
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It is passed verbatim to the datasource, so you must use the case that the
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datasource is expecting.
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The 'defaultDir' member is used with file based datasources (i.e. dBase,
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FoxPro, text files). It contains a full path to the location where the
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data table or file is located. When setting this value, use forward
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slashes '/' rather than backslashes '\' to avoid compatibility differences
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between ODBC drivers.
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The other fields are currently unused. The intent of these fields are that
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they will be used to write our own ODBC Administrator type program that will
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work on both MSW and Un*x systems, regardless of the datasource. Very little
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work has been done on this to date.
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{\bf Get a Datasource Connection}
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There are two methods of establishing a connection to a datasource. You
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may either manually create your own wxDb instance and open the connection,
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or you may use the caching functions provided with the wxODBC classes to
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create/maintain/delete the connections.
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Regardless of which method you use, you must first have a fully populated
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wxDbConnectInf object. In the wxDbConnectInf instance, provide a valid
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Dns, Uid, and AuthStr (along with a 'defaultDir' if necessary). Before
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using this though, you must allocate an environment handle to the 'Henv'
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member.
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\begin{verbatim}
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wxDbConnectInf DbConnectInf;
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DbConnectInf.SetDsn("MyDSN");
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DbConnectInf.SetUserID("MyUserName");
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DbConnectInf.SetPassword("MyPassword");
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DbConnectInf.SetDefaultDir("");
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\end{verbatim}
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To allocate an environment handle for the ODBC connection to use, the
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wxDbConnectInf class has a datasource independent method for creating
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the necessary handle:
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\begin{verbatim}
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if (DbConnectInf.AllocHenv())
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{
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wxMessageBox("Unable to allocate an ODBC environment handle",
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"DB CONNECTION ERROR", wxOK | wxICON_EXCLAMATION);
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return;
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}
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\end{verbatim}
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When the wxDbConnectInf::AllocHenv() function is called successfully, a
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value of true will be returned. A value of false means allocation failed,
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and the handle will be undefined.
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A shorter form of doing the above steps is encapsulated into the
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long form of the constructor for wxDbConnectInf.
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\begin{verbatim}
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wxDbConnectInf *DbConnectInf;
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DbConnectInf = new wxDbConnectInf(NULL, "MyDSN", "MyUserName",
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"MyPassword", "");
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\end{verbatim}
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This shorthand form of initializing the constructor passes a NULL for the SQL
|
|
environment handle, telling the constructor to allocate a handle during
|
|
construction. This handle is also managed for the life of wxDbConnectInf
|
|
instance, and is freed automatically upon destruction of the instance.
|
|
|
|
Once the wxDbConnectInf instance is initialized, you are ready to
|
|
connect to the datasource.
|
|
|
|
To manually create datasource connections, you must create a wxDb
|
|
instance, and then open it.
|
|
|
|
\begin{verbatim}
|
|
wxDb *db = new wxDb(DbConnectInf->GetHenv());
|
|
|
|
opened = db->Open(DbConnectInf);
|
|
\end{verbatim}
|
|
|
|
The first line does the house keeping needed to initialize all
|
|
the members of the wxDb class. The second line actually sends the request
|
|
to the ODBC driver to open a connection to its associated datasource using
|
|
the parameters supplied in the call to \helpref{wxDb::Open}{wxdbopen}.
|
|
|
|
A more advanced form of opening a connection is to use the connection
|
|
caching functions that are included with the wxODBC classes. The caching
|
|
mechanisms perform the same functions as the manual approach to opening a
|
|
connection, but they also manage each connection they have created,
|
|
re-using them and cleaning them up when they are closed, without you
|
|
needing to do the coding.
|
|
|
|
To use the caching function \helpref{wxDbGetConnection}{wxdbfunctions} to get
|
|
a connection to a datasource, simply call it with a single parameter of the
|
|
type wxDbConnectInf:
|
|
|
|
\begin{verbatim}
|
|
db = wxDbGetConnection(DbConnectInf);
|
|
\end{verbatim}
|
|
|
|
The wxDb pointer that is returned is both initialized and opened. If
|
|
something failed in creating or opening the connection, the return value
|
|
from \helpref{wxDbGetConnection}{wxdbfunctions} will be NULL.
|
|
|
|
The connection that is returned is either a new connection, or it is a
|
|
"free" connection from the cache of connections that the class maintains
|
|
that was no longer in use. Any wxDb instance created with a call to
|
|
\helpref{wxDbGetConnection}{wxdbfunctions} is recorded in a linked list of established
|
|
connections. When a program is finished with a connection, a call to
|
|
\helpref{wxDbFreeConnection}{wxdbfunctions} is made, and the datasource
|
|
connection will then be tagged as FREE, making it available for the next
|
|
call to \helpref{wxDbGetConnection}{wxdbfunctions} that needs a connection
|
|
using the same connection information (Dsn, Uid, AuthStr). The cached
|
|
connections remain cached until a call to \helpref{wxDbCloseConnections}{wxdbfunctions} is made,
|
|
at which time all cached connections are closed and deleted.
|
|
|
|
Besides the obvious advantage of using the single command caching routine to
|
|
obtain a datasource connection, using cached connections can be quite a
|
|
performance boost as well. Each time that a new connection is created
|
|
(not retrieved from the cache of free connections), the wxODBC classes
|
|
perform many queries against the datasource to determine the datasource's
|
|
datatypes and other fundamental behaviours. Depending on the hardware,
|
|
network bandwidth, and datasource speed, this can in some cases take a
|
|
few seconds to establish the new connection (with well-balanced systems,
|
|
it should only be a fraction of a second). Re-using already established
|
|
datasource connections rather than creating/deleting, creating/deleting
|
|
connections can be quite a time-saver.
|
|
|
|
Another time-saver is the "copy connection" features of both
|
|
\helpref{wxDb::Open}{wxdbopen} and \helpref{wxDbGetConnection}{wxdbfunctions}.
|
|
If manually creating a wxDb instance and opening it, you must pass an existing
|
|
connection to the \helpref{wxDb::Open}{wxdbopen} function yourself to gain the performance
|
|
benefit of copying existing connection settings. The
|
|
\helpref{wxDbGetConnection}{wxdbfunctions} function automatically does this
|
|
for you, checking the Dsn, Uid, and AuthStr parameters when you request
|
|
a connection for any existing connections that use those same settings.
|
|
If one is found, \helpref{wxDbGetConnection}{wxdbfunctions} copies the datasource settings for
|
|
datatypes and other datasource specific information that was previously
|
|
queried, rather than re-querying the datasource for all those same settings.
|
|
|
|
One final note on creating a connection. When a connection is created, it
|
|
will default to only allowing cursor scrolling to be either forward only,
|
|
or both backward and forward scrolling. The default behavior is
|
|
determined by the setting {\tt wxODBC\_FWD\_ONLY\_CURSORS} in setup.h when you
|
|
compile the wxWidgets library. The library default is to only support
|
|
forward scrolling cursors only, though this can be overridden by parameters
|
|
for wxDb() constructor or the \helpref{wxDbGetConnection}{wxdbfunctions}
|
|
function. All datasources and ODBC drivers must support forward scrolling
|
|
cursors. Many datasources support backward scrolling cursors, and many
|
|
ODBC drivers support backward scrolling cursors. Before planning on using
|
|
backward scrolling cursors, you must be certain that both your datasource
|
|
and ODBC driver fully support backward scrolling cursors. See the small
|
|
blurb about "Scrolling cursors" in the definitions at the beginning of
|
|
this overview, or other details of setting the cursor behavior in the wxDb
|
|
class documentation.
|
|
|
|
{\bf Create Table Definition}
|
|
|
|
Data can be accessed in a datasource's tables directly through various
|
|
functions of the wxDb class (see \helpref{wxDb::GetData}{wxdbgetdata}). But to make life much
|
|
simpler, the wxDbTable class encapsulates all of the SQL specific API calls
|
|
that would be necessary to do this, wrapping it in an intuitive class of APIs.
|
|
|
|
The first step in accessing data in a datasource's tables via the wxDbTable
|
|
class is to create a wxDbTable instance.
|
|
|
|
\begin{verbatim}
|
|
table = new wxDbTable(db, tableName, numTableColumns, "",
|
|
!wxDB_QUERY_ONLY, "");
|
|
\end{verbatim}
|
|
|
|
When you create the instance, you indicate the previously established
|
|
datasource connection to be used to access the table, the name of the
|
|
primary table that is to be accessed with the datasource's tables, how many
|
|
columns of each row are going to be returned, the name of the view of the
|
|
table that will actually be used to query against (works with Oracle only
|
|
at this time), whether the data returned is for query purposes only, and
|
|
finally the path to the table, if different than the path specified when
|
|
connecting to the datasource.
|
|
|
|
Each of the above parameters are described in detail in the wxDbTable
|
|
class' description, but one special note here about the fifth
|
|
parameter - the queryOnly setting. If a wxDbTable instance is created as
|
|
{\tt wxDB\_QUERY\_ONLY}, then no inserts/deletes/updates can be performed
|
|
using this instance of the wxDbTable. Any calls to \helpref{wxDb::CommitTrans}{wxdbcommittrans}
|
|
or \helpref{wxDb::RollbackTrans}{wxdbrollbacktrans} against the datasource
|
|
connection used by this wxDbTable instance are ignored by this instance. If
|
|
the wxDbTable instance is created with {\tt !wxDB\_QUERY\_ONLY} as shown above,
|
|
then all the cursors and other overhead associated with being able to
|
|
insert/update/delete data in the table are created, and thereby those
|
|
operations can then be performed against the associated table with this
|
|
wxDbTable instance.
|
|
|
|
If a table is to be accessed via a wxDbTable instance, and the table will
|
|
only be read from, not written to, there is a performance benefit (not as
|
|
many cursors need to be maintained/updated, hence speeding up access times),
|
|
as well as a resource savings due to fewer cursors being created for the
|
|
wxDbTable instance. Also, with some datasources, the number of
|
|
simultaneous cursors is limited.
|
|
|
|
When defining the columns to be retrievable by the wxDbTable instance, you
|
|
can specify anywhere from one column up to all columns in the table.
|
|
|
|
\begin{verbatim}
|
|
table->SetColDefs(0, "FIRST_NAME", DB_DATA_TYPE_VARCHAR, FirstName,
|
|
SQL_C_WXCHAR, sizeof(FirstName), true, true);
|
|
table->SetColDefs(1, "LAST_NAME", DB_DATA_TYPE_VARCHAR, LastName,
|
|
SQL_C_WXCHAR, sizeof(LastName), true, true);
|
|
\end{verbatim}
|
|
|
|
Notice that column definitions start at index 0 and go up to one less than
|
|
the number of columns specified when the wxDbTable instance was created
|
|
(in this example, two columns - one with index 0, one with index 1).
|
|
|
|
The above lines of code "bind" the datasource columns specified to the
|
|
memory variables in the client application. So when the application
|
|
makes a call to \helpref{wxDbTable::GetNext}{wxdbtablegetnext} (or any other function that retrieves
|
|
data from the result set), the variables that are bound to the columns will
|
|
have the column value stored into them. See the
|
|
\helpref{wxDbTable::SetColDefs}{wxdbtablesetcoldefs}
|
|
class documentation for more details on all the parameters for this function.
|
|
|
|
The bound memory variables have undefined data in them until a call to a
|
|
function that retrieves data from a result set is made
|
|
(e.g. \helpref{wxDbTable::GetNext}{wxdbtablegetnext},
|
|
\helpref{wxDbTable::GetPrev}{wxdbtablegetprev}, etc). The variables are not
|
|
initialized to any data by the wxODBC classes, and they still contain
|
|
undefined data after a call to \helpref{wxDbTable::Query}{wxdbtablequery}. Only
|
|
after a successful call to one of the ::GetXxxx() functions is made do the
|
|
variables contain valid data.
|
|
|
|
It is not necessary to define column definitions for columns whose data is
|
|
not going to be returned to the client. For example, if you want to query
|
|
the datasource for all users with a first name of 'GEORGE', but you only want
|
|
the list of last names associated with those rows (why return the FIRST\_NAME
|
|
column every time when you already know it is 'GEORGE'), you would only have
|
|
needed to define one column above.
|
|
|
|
You may have as many wxDbTable instances accessing the same table using the
|
|
same wxDb instance as you desire. There is no limit imposed by the classes
|
|
on this. All datasources supported (so far) also have no limitations on this.
|
|
|
|
{\bf Open the table}
|
|
|
|
Opening the table is not technically doing anything with the datasource
|
|
itself. Calling \helpref{wxDbTable::Open}{wxdbtableopen} simply does all the
|
|
housekeeping of checking that the specified table exists, that the current
|
|
connected user has at least SELECT privileges for accessing the table,
|
|
setting up the requisite cursors, binding columns and cursors, and
|
|
constructing the default INSERT statement that is used when a new row is
|
|
inserted into the table (non-wxDB\_QUERY\_ONLY tables only).
|
|
|
|
\begin{verbatim}
|
|
if (!table->Open())
|
|
{
|
|
// An error occurred opening (setting up) the table
|
|
}
|
|
\end{verbatim}
|
|
|
|
The only reason that a call to \helpref{wxDbTable::Open}{wxdbtableopen} is likely to fail is if the
|
|
user has insufficient privileges to even SELECT the table. Other problems
|
|
could occur, such as being unable to bind columns, but these other reason
|
|
point to some lack of resource (like memory). Any errors generated
|
|
internally in the \helpref{wxDbTable::Open}{wxdbtableopen} function are logged to the error log
|
|
if SQL logging is turned on for the classes.
|
|
|
|
{\bf Use the table}
|
|
|
|
To use the table and the definitions that are now set up, we must first
|
|
define what data we want the datasource to collect in to a result set, tell
|
|
it where to get the data from, and in which sequence we want the data returned.
|
|
|
|
\begin{verbatim}
|
|
// the WHERE clause limits/specifies which rows in the table
|
|
// are to be returned in the result set
|
|
table->SetWhereClause("FIRST_NAME = 'GEORGE'");
|
|
|
|
// Result set will be sorted in ascending alphabetical
|
|
// order on the data in the 'LAST_NAME' column of each row
|
|
// If the same last name is in the table for two rows,
|
|
// sub-sort on the 'AGE' column
|
|
table->SetOrderByClause("LAST_NAME, AGE");
|
|
|
|
// No other tables (joins) are used for this query
|
|
table->SetFromClause("");
|
|
\end{verbatim}
|
|
|
|
The above lines will be used to tell the datasource to return in the result
|
|
all the rows in the table whose column "FIRST\_NAME" contains the name
|
|
'GEORGE' (note the required use of the single quote around the string
|
|
literal) and that the result set will return the rows sorted by ascending
|
|
last names (ascending is the default, and can be overridden with the
|
|
"DESC" keyword for datasources that support it - "LAST\_NAME DESC").
|
|
|
|
Specifying a blank WHERE clause will result in the result set containing
|
|
all rows in the datasource.
|
|
|
|
Specifying a blank ORDERBY clause means that the datasource will return
|
|
the result set in whatever sequence it encounters rows which match the
|
|
selection criteria. What this sequence is can be hard to determine.
|
|
Typically it depends on the index that the datasource used to find the
|
|
rows which match the WHERE criteria. BEWARE - relying on the datasource
|
|
to return data in a certain sequence when you have not provided an ORDERBY
|
|
clause will eventually cause a problem for your program. Databases can be
|
|
tuned to be COST-based, SPEED-based, or some other basis for how it gets
|
|
your result set. In short, if you need your result set returned in a
|
|
specific sequence, ask for it that way by providing an ORDERBY clause.
|
|
|
|
Using an ORDERBY clause can be a performance hit, as the database must
|
|
sort the items before making the result set available to the client.
|
|
Creating efficient indexes that cause the data to be "found" in the correct
|
|
ORDERBY sequence can be a big performance benefit. Also, in the large
|
|
majority of cases, the database will be able to sort the records faster
|
|
than your application can read all the records in (unsorted) and then sort
|
|
them. Let the database do the work for you!
|
|
|
|
Notice in the example above, a column that is not included in the bound
|
|
data columns ('AGE') will be used to sub-sort the result set.
|
|
|
|
The FROM clause in this example is blanked, as we are not going to be
|
|
performing any table joins with this simple query. When the FROM clause
|
|
is blank, it is assumed that all columns referenced are coming from
|
|
the default table for the wxDbTable instance.
|
|
|
|
After the selection criteria have been specified, the program can now
|
|
ask the datasource to perform the search and create a result set that
|
|
can be retrieved:
|
|
|
|
\begin{verbatim}
|
|
// Instruct the datasource to perform a query based on the
|
|
// criteria specified above in the where/orderBy/from clauses.
|
|
if (!table->Query())
|
|
{
|
|
// An error occurred performing the query
|
|
}
|
|
\end{verbatim}
|
|
|
|
Typically, when an error occurs when calling \helpref{wxDbTable::Query}{wxdbtablequery}, it is a
|
|
syntax problem in the WHERE clause that was specified. The exact SQL
|
|
(datasource-specific) reason for what caused the failure of \helpref{wxDbTable::Query}{wxdbtablequery}
|
|
(and all other operations against the datasource can be found by
|
|
parsing the table's database connection's "errorList[]" array member for
|
|
the stored text of the error.
|
|
|
|
When the \helpref{wxDbTable::Query}{wxdbtablequery} returns true, the
|
|
database was able to successfully complete the requested query using the
|
|
provided criteria. This does not mean that there are any rows in the
|
|
result set, it just mean that the query was successful.
|
|
|
|
\normalbox{IMPORTANT: The result created by the call to
|
|
\helpref{wxDbTable::Query}{wxdbtablequery} can take one of two forms. It is
|
|
either a snapshot of the data at the exact moment that the database
|
|
determined the record matched the search criteria, or it is a pointer to
|
|
the row that matched the selection criteria. Which form of behavior is
|
|
datasource dependent. If it is a snapshot, the data may have changed
|
|
since the result set was constructed, so beware if your datasource
|
|
uses snapshots and call \helpref{wxDbTable::Refresh}{wxdbtablerefresh}. Most larger brand databases
|
|
do not use snapshots, but it is important to mention so that your application
|
|
can handle it properly if your datasource does.}
|
|
|
|
To retrieve the data, one of the data fetching routines must be used to
|
|
request a row from the result set, and to store the data from the result
|
|
set into the bound memory variables. After \helpref{wxDbTable::Query}{wxdbtablequery}
|
|
has completed successfully, the default/current cursor is placed so it
|
|
is pointing just before the first record in the result set. If the
|
|
result set is empty (no rows matched the criteria), then any calls to
|
|
retrieve data from the result set will return false.
|
|
|
|
\begin{verbatim}
|
|
wxString msg;
|
|
|
|
while (table->GetNext())
|
|
{
|
|
msg.Printf("Row #%lu -- First Name : %s Last Name is %s",
|
|
table->GetRowNum(), FirstName, LastName);
|
|
wxMessageBox(msg, "Data", wxOK | wxICON_INFORMATION, NULL);
|
|
}
|
|
\end{verbatim}
|
|
|
|
The sample code above will read the next record in the result set repeatedly
|
|
until the end of the result set has been reached. The first time that
|
|
\helpref{wxDbTable::GetNext}{wxdbtablegetnext} is called right after the successful
|
|
call to \helpref{wxDbTable::Query}{wxdbtablequery}, it actually returns the first record
|
|
in the result set.
|
|
|
|
When \helpref{wxDbTable::GetNext}{wxdbtablegetnext} is called and there are
|
|
no rows remaining in the result set after the current cursor position,
|
|
\helpref{wxDbTable::GetNext}{wxdbtablegetnext} (as well as all the other
|
|
wxDbTable::GetXxxxx() functions) will return false.
|
|
|
|
{\bf Close the table}
|
|
|
|
When the program is done using a wxDbTable instance, it is as simple as
|
|
deleting the table pointer (or if declared statically, letting the
|
|
variable go out of scope). Typically the default destructor will take
|
|
care of all that is required for cleaning up the wxDbTable instance.
|
|
|
|
\begin{verbatim}
|
|
if (table)
|
|
{
|
|
delete table;
|
|
table = NULL;
|
|
}
|
|
\end{verbatim}
|
|
|
|
Deleting a wxDbTable instance releases all of its cursors, deletes the
|
|
column definitions and frees the SQL environment handles used by the
|
|
table (but not the environment handle used by the datasource connection
|
|
that the wxDbTable instance was using).
|
|
|
|
{\bf Close the datasource connection}
|
|
|
|
After all tables that have been using a datasource connection have been
|
|
closed (this can be verified by calling \helpref{wxDb::GetTableCount}{wxdbgettablecount}
|
|
and checking that it returns 0), then you may close the datasource
|
|
connection. The method of doing this is dependent on whether the
|
|
non-caching or caching method was used to obtain the datasource connection.
|
|
|
|
If the datasource connection was created manually (non-cached), closing the
|
|
connection is done like this:
|
|
|
|
\begin{verbatim}
|
|
if (db)
|
|
{
|
|
db->Close();
|
|
delete db;
|
|
db = NULL;
|
|
}
|
|
\end{verbatim}
|
|
|
|
If the program used the \helpref{wxDbGetConnection}{wxdbfunctions} function to get a datasource
|
|
connection, the following is the code that should be used to free the
|
|
connection(s):
|
|
|
|
\begin{verbatim}
|
|
if (db)
|
|
{
|
|
wxDbFreeConnection(db);
|
|
db = NULL;
|
|
}
|
|
\end{verbatim}
|
|
|
|
Note that the above code just frees the connection so that it can be
|
|
re-used on the next call the \helpref{wxDbGetConnection}{wxdbfunctions}. To actually dispose
|
|
of the connection, releasing all of its resources (other than the
|
|
environment handle), do the following:
|
|
|
|
\begin{verbatim}
|
|
wxDbCloseConnections();
|
|
\end{verbatim}
|
|
|
|
{\bf Release the ODBC environment handle}
|
|
|
|
Once all of the connections that used the ODBC environment handle (in
|
|
this example it was stored in "DbConnectInf.Henv") have been closed, then
|
|
it is safe to release the environment handle:
|
|
|
|
\begin{verbatim}
|
|
DbConnectInf->FreeHenv();
|
|
\end{verbatim}
|
|
|
|
Or, if the long form of the constructor was used and the constructor was allowed
|
|
to allocate its own SQL environment handle, leaving scope or destruction of the
|
|
wxDbConnectInf will free the handle automatically.
|
|
|
|
\begin{verbatim}
|
|
delete DbConnectInf;
|
|
\end{verbatim}
|
|
|
|
\normalbox{Remember to never release this environment handle if there are any
|
|
connections still using the handle.}
|
|
|
|
\subsection{wxODBC - Known Issues}\label{wxodbcknownissues}
|
|
|
|
As with creating wxWidgets, writing the wxODBC classes was not the simple
|
|
task of writing an application to run on a single type of computer system.
|
|
The classes need to be cross-platform for different operating systems, and
|
|
they also needed to take in to account different database manufacturers and
|
|
different ODBC driver manufacturers. Because of all the possible combinations
|
|
of OS/database/drivers, it is impossible to say that these classes will work
|
|
perfectly with datasource ABC, ODBC driver XYZ, on platform LMN. You may run
|
|
in to some incompatibilities or unsupported features when moving your
|
|
application from one environment to another. But that is what makes
|
|
cross-platform programming fun. It is also pinpoints one of the great
|
|
things about open source software. It can evolve!
|
|
|
|
The most common difference between different database/ODBC driver
|
|
manufacturers in regards to these wxODBC classes is the lack of
|
|
standard error codes being returned to the calling program. Sometimes
|
|
manufacturers have even changed the error codes between versions of
|
|
their databases/drivers.
|
|
|
|
In all the tested databases, every effort has been made to determine
|
|
the correct error codes and handle them in the class members that need
|
|
to check for specific error codes (such as TABLE DOES NOT EXIST when
|
|
you try to open a table that has not been created yet). Adding support
|
|
for additional databases in the future requires adding an entry for the
|
|
database in the \helpref{wxDb::Dbms}{wxdbdbms} function, and then handling any error codes
|
|
returned by the datasource that do not match the expected values.
|
|
|
|
{\bf Databases}
|
|
|
|
Following is a list of known issues and incompatibilities that the
|
|
wxODBC classes have between different datasources. An up to date
|
|
listing of known issues can be seen in the comments of the source
|
|
for \helpref{wxDb::Dbms}{wxdbdbms}.
|
|
|
|
{\it ORACLE}
|
|
\begin{itemize}\itemsep=0pt
|
|
\item Currently the only database supported by the wxODBC classes to support VIEWS
|
|
\end{itemize}
|
|
|
|
{\it DBASE}
|
|
|
|
NOTE: dBase is not a true ODBC datasource. You only have access to as much
|
|
functionality as the driver can emulate.
|
|
|
|
\begin{itemize}\itemsep=0pt
|
|
\item Does not support the SQL\_TIMESTAMP structure
|
|
\item Supports only one cursor and one connect (apparently? with Microsoft driver only?)
|
|
\item Does not automatically create the primary index if the 'keyField' param of SetColDef is true. The user must create ALL indexes from their program with calls to \helpref{wxDbTable::CreateIndex}{wxdbtablecreateindex}
|
|
\item Table names can only be 8 characters long
|
|
\item Column names can only be 10 characters long
|
|
\item Currently cannot CREATE a dBase table - bug or limitation of the drivers used??
|
|
\item Currently cannot insert rows that have integer columns - bug??
|
|
\end{itemize}
|
|
|
|
{\it SYBASE (all)}
|
|
\begin{itemize}\itemsep=0pt
|
|
\item To lock a record during QUERY functions, the reserved word 'HOLDLOCK' must be added after every table name involved in the query/join if that table's matching record(s) are to be locked
|
|
\item Ignores the keywords 'FOR UPDATE'. Use the HOLDLOCK functionality described above
|
|
\end{itemize}
|
|
|
|
{\it SYBASE (Enterprise)}
|
|
\begin{itemize}\itemsep=0pt
|
|
\item If a column is part of the Primary Key, the column cannot be NULL
|
|
\item Maximum row size is somewhere in the neighborhood of 1920 bytes
|
|
\end{itemize}
|
|
|
|
{\it mySQL}
|
|
\begin{itemize}\itemsep=0pt
|
|
\item If a column is part of the Primary Key, the column cannot be NULL.
|
|
\item Cannot support selecting for update [\helpref{wxDbTable::CanSelectForUpdate}{wxdbtablecanselectforupdate}]. Always returns false.
|
|
\item Columns that are part of primary or secondary keys must be defined as being NOT NULL when they are created. Some code is added in \helpref{wxDbTable::CreateIndex}{wxdbtablecreateindex} to try to adjust the column definition if it is not defined correctly, but it is experimental (as of wxWidgets v2.2.1)
|
|
\item Does not support sub-queries in SQL statements
|
|
\end{itemize}
|
|
|
|
{\it POSTGRES}
|
|
\begin{itemize}\itemsep=0pt
|
|
\item Does not support the keywords 'ASC' or 'DESC' as of release v6.5.0
|
|
\item Does not support sub-queries in SQL statements
|
|
\end{itemize}
|
|
|
|
{\it DB2}
|
|
\begin{itemize}\itemsep=0pt
|
|
\item Columns which are part of a primary key must be declared as NOT NULL
|
|
\end{itemize}
|
|
|
|
{\bf UNICODE with wxODBC classes}
|
|
|
|
As of v2.6 of wxWidgets, the wxODBC classes now fully support the compilation
|
|
and use of the classes in a Unicode build of wxWidgets, assuming the compiler
|
|
and OS on which the program will be compiled/run is Unicode capable.
|
|
|
|
The one major difference in writing code that can be compiled in either
|
|
unicode or non-unicode builds that is specific to the wxODBC classes is to
|
|
use the SQL_C_WXCHAR datatype for string columns rather than SQL_C_CHAR or
|
|
SQL_C_WCHAR.
|
|
|
|
\subsection{wxODBC - Sample Code}\label{wxodbcsamplecode1}
|
|
|
|
Simplest example of establishing/opening a connection to an ODBC datasource,
|
|
binding variables to the columns for read/write usage, opening an
|
|
existing table in the datasource, inserting a record, setting query parameters
|
|
(where/orderBy/from), querying the datasource, reading each row of the
|
|
result set, deleting a record, releasing the connection, then cleaning up.
|
|
|
|
NOTE: Very basic error handling is shown here, to reduce the size of the
|
|
code and to make it more easily readable. The HandleError() function uses the wxDbLogExtendedErrorMsg() function for retrieving database error messages.
|
|
|
|
\begin{verbatim}
|
|
// ----------------------------------------------------------------------------
|
|
// HEADERS
|
|
// ----------------------------------------------------------------------------
|
|
#include "wx/log.h" // #included to enable output of messages only
|
|
#include "wx/dbtable.h"
|
|
|
|
// ----------------------------------------------------------------------------
|
|
// FUNCTION USED FOR HANDLING/DISPLAYING ERRORS
|
|
// ----------------------------------------------------------------------------
|
|
// Very generic error handling function.
|
|
// If a connection to the database is passed in, then we retrieve all the
|
|
// database errors for the connection and add them to the displayed message
|
|
int HandleError(wxString errmsg, wxDb *pDb=NULL)
|
|
{
|
|
// Retrieve all the error message for the errors that occurred
|
|
wxString allErrors;
|
|
if (!pDb == NULL)
|
|
// Get the database errors and append them to the error message
|
|
allErrors = wxDbLogExtendedErrorMsg(errmsg.c_str(), pDb, 0, 0);
|
|
else
|
|
allErrors = errmsg;
|
|
|
|
// Do whatever you wish with the error message here
|
|
// wxLogDebug() is called inside wxDbLogExtendedErrorMsg() so this
|
|
// console program will show the errors in the console window,
|
|
// but these lines will show the errors in RELEASE builds also
|
|
wxFprintf(stderr, wxT("\n%s\n"), allErrors.c_str());
|
|
fflush(stderr);
|
|
|
|
return 1;
|
|
}
|
|
|
|
|
|
// ----------------------------------------------------------------------------
|
|
// entry point
|
|
// ----------------------------------------------------------------------------
|
|
int main(int argc, char **argv)
|
|
{
|
|
wxDbConnectInf *DbConnectInf = NULL; // DB connection information
|
|
|
|
wxDb *db = NULL; // Database connection
|
|
|
|
wxDbTable *table = NULL; // Data table to access
|
|
const wxChar tableName[] = wxT("USERS"); // Name of database table
|
|
const UWORD numTableColumns = 2; // Number table columns
|
|
wxChar FirstName[50+1]; // column data: "FIRST_NAME"
|
|
wxChar LastName[50+1]; // column data: "LAST_NAME"
|
|
|
|
wxString msg; // Used for display messages
|
|
|
|
// -----------------------------------------------------------------------
|
|
// DEFINE THE CONNECTION HANDLE FOR THE DATABASE
|
|
// -----------------------------------------------------------------------
|
|
DbConnectInf = new wxDbConnectInf(NULL,
|
|
wxT("CONTACTS-SqlServer"),
|
|
wxT("sa"),
|
|
wxT("abk"));
|
|
|
|
// Error checking....
|
|
if (!DbConnectInf || !DbConnectInf->GetHenv())
|
|
{
|
|
return HandleError(wxT("DB ENV ERROR: Cannot allocate ODBC env handle"));
|
|
}
|
|
|
|
|
|
// -----------------------------------------------------------------------
|
|
// GET A DATABASE CONNECTION
|
|
// -----------------------------------------------------------------------
|
|
db = wxDbGetConnection(DbConnectInf);
|
|
|
|
if (!db)
|
|
{
|
|
return HandleError(wxT("CONNECTION ERROR - Cannot get DB connection"));
|
|
}
|
|
|
|
|
|
// -----------------------------------------------------------------------
|
|
// DEFINE THE TABLE, AND THE COLUMNS THAT WILL BE ACCESSED
|
|
// -----------------------------------------------------------------------
|
|
table = new wxDbTable(db, tableName, numTableColumns, wxT(""),
|
|
!wxDB_QUERY_ONLY, wxT(""));
|
|
//
|
|
// Bind the columns that you wish to retrieve. Note that there must be
|
|
// 'numTableColumns' calls to SetColDefs(), to match the wxDbTable def
|
|
//
|
|
// Not all columns need to be bound, only columns whose values are to be
|
|
// returned back to the client.
|
|
//
|
|
table->SetColDefs(0, wxT("FIRST_NAME"), DB_DATA_TYPE_VARCHAR, FirstName,
|
|
SQL_C_WXCHAR, sizeof(FirstName), true, true);
|
|
table->SetColDefs(1, wxT("LAST_NAME"), DB_DATA_TYPE_VARCHAR, LastName,
|
|
SQL_C_WXCHAR, sizeof(LastName), true, true);
|
|
|
|
|
|
// -----------------------------------------------------------------------
|
|
// CREATE (or RECREATE) THE TABLE IN THE DATABASE
|
|
// -----------------------------------------------------------------------
|
|
if (!table->CreateTable(true)) //NOTE: No CommitTrans is required
|
|
{
|
|
return HandleError(wxT("TABLE CREATION ERROR: "), table->GetDb());
|
|
}
|
|
|
|
|
|
// -----------------------------------------------------------------------
|
|
// OPEN THE TABLE FOR ACCESS
|
|
// -----------------------------------------------------------------------
|
|
if (!table->Open())
|
|
{
|
|
return HandleError(wxT("TABLE OPEN ERROR: "), table->GetDb());
|
|
}
|
|
|
|
|
|
// -----------------------------------------------------------------------
|
|
// INSERT A NEW ROW INTO THE TABLE
|
|
// -----------------------------------------------------------------------
|
|
wxStrcpy(FirstName, wxT("JULIAN"));
|
|
wxStrcpy(LastName, wxT("SMART"));
|
|
if (!table->Insert())
|
|
{
|
|
return HandleError(wxT("INSERTION ERROR: "), table->GetDb());
|
|
}
|
|
|
|
// Must commit the insert to write the data to the DB
|
|
table->GetDb()->CommitTrans();
|
|
|
|
|
|
// -----------------------------------------------------------------------
|
|
// RETRIEVE ROWS FROM THE TABLE BASED ON SUPPLIED CRITERIA
|
|
// -----------------------------------------------------------------------
|
|
// Set the WHERE clause to limit the result set to return
|
|
// all rows that have a value of 'JULIAN' in the FIRST_NAME
|
|
// column of the table.
|
|
table->SetWhereClause(wxT("FIRST_NAME = 'JULIAN'"));
|
|
|
|
// Result set will be sorted in ascending alphabetical
|
|
// order on the data in the 'LAST_NAME' column of each row
|
|
table->SetOrderByClause(wxT("LAST_NAME"));
|
|
|
|
// No other tables (joins) are used for this query
|
|
table->SetFromClause(wxT(""));
|
|
|
|
// Instruct the datasource to perform a query based on the
|
|
// criteria specified above in the where/orderBy/from clauses.
|
|
if (!table->Query())
|
|
{
|
|
return HandleError(wxT("QUERY ERROR: "), table->GetDb());
|
|
}
|
|
|
|
// Loop through all rows matching the query criteria until
|
|
// there are no more records to read
|
|
while (table->GetNext())
|
|
{
|
|
msg.Printf(wxT("Row #%lu -- First Name : %s Last Name is %s"),
|
|
table->GetRowNum(), FirstName, LastName);
|
|
|
|
// Code to display 'msg' here
|
|
wxLogMessage(wxT("\n%s\n"), msg.c_str());
|
|
}
|
|
|
|
|
|
// -----------------------------------------------------------------------
|
|
// DELETE A ROW FROM THE TABLE
|
|
// -----------------------------------------------------------------------
|
|
// Select the row which has FIRST_NAME of 'JULIAN' and LAST_NAME
|
|
// of 'SMART', then delete the retrieved row
|
|
//
|
|
if (!table->DeleteWhere(wxT("FIRST_NAME = 'JULIAN' and LAST_NAME = 'SMART'")))
|
|
{
|
|
return HandleError(wxT("DELETION ERROR: "), table->GetDb());
|
|
}
|
|
|
|
// Must commit the deletion to the database
|
|
table->GetDb()->CommitTrans();
|
|
|
|
|
|
// -----------------------------------------------------------------------
|
|
// TAKE CARE OF THE ODBC CLASS INSTANCES THAT WERE BEING USED
|
|
// -----------------------------------------------------------------------
|
|
// If the wxDbTable instance was successfully created
|
|
// then delete it as we are done with it now.
|
|
wxDELETE(table);
|
|
|
|
// Free the cached connection
|
|
// (meaning release it back in to the cache of datasource
|
|
// connections) for the next time a call to wxDbGetConnection()
|
|
// is made.
|
|
wxDbFreeConnection(db);
|
|
db = NULL;
|
|
|
|
|
|
// -----------------------------------------------------------------------
|
|
// CLEANUP BEFORE EXITING APP
|
|
// -----------------------------------------------------------------------
|
|
// The program is now ending, so we need to close
|
|
// any cached connections that are still being
|
|
// maintained.
|
|
wxDbCloseConnections();
|
|
|
|
// Release the environment handle that was created
|
|
// for use with the ODBC datasource connections
|
|
wxDELETE(DbConnectInf);
|
|
|
|
wxUnusedVar(argc); // Here just to prevent compiler warnings
|
|
wxUnusedVar(argv); // Here just to prevent compiler warnings
|
|
|
|
return 0;
|
|
}
|
|
\end{verbatim}
|
|
|
|
\subsection{A selection of SQL commands}\label{sqlcommands}
|
|
|
|
The following is a very brief description of some common SQL commands, with
|
|
examples.
|
|
|
|
\wxheading{See also}
|
|
|
|
\helpref{Database classes overview}{odbcoverview}
|
|
|
|
\subsubsection{Create}\label{odbccreateexample}
|
|
|
|
Creates a table.
|
|
|
|
Example:
|
|
|
|
\begin{verbatim}
|
|
CREATE TABLE Book
|
|
(BookNumber INTEGER PRIMARY KEY
|
|
, CategoryCode CHAR(2) DEFAULT 'RO' NOT NULL
|
|
, Title VARCHAR(100) UNIQUE
|
|
, NumberOfPages SMALLINT
|
|
, RetailPriceAmount NUMERIC(5,2)
|
|
)
|
|
\end{verbatim}
|
|
|
|
\subsubsection{Insert}\label{odbcinsertexample}
|
|
|
|
Inserts records into a table.
|
|
|
|
Example:
|
|
|
|
\begin{verbatim}
|
|
INSERT INTO Book
|
|
(BookNumber, CategoryCode, Title)
|
|
VALUES(5, 'HR', 'The Lark Ascending')
|
|
\end{verbatim}
|
|
|
|
\subsubsection{Select}\label{odbcselectexample}
|
|
|
|
The Select operation retrieves rows and columns from a table. The criteria
|
|
for selection and the columns returned may be specified.
|
|
|
|
Examples:
|
|
|
|
{\tt SELECT * FROM Book}
|
|
|
|
Selects all rows and columns from table Book.
|
|
|
|
{\tt SELECT Title, RetailPriceAmount FROM Book WHERE RetailPriceAmount > 20.0}
|
|
|
|
Selects columns Title and RetailPriceAmount from table Book, returning only
|
|
the rows that match the WHERE clause.
|
|
|
|
{\tt SELECT * FROM Book WHERE CatCode = 'LL' OR CatCode = 'RR'}
|
|
|
|
Selects all columns from table Book, returning only
|
|
the rows that match the WHERE clause.
|
|
|
|
{\tt SELECT * FROM Book WHERE CatCode IS NULL}
|
|
|
|
Selects all columns from table Book, returning only rows where the CatCode column
|
|
is NULL.
|
|
|
|
{\tt SELECT * FROM Book ORDER BY Title}
|
|
|
|
Selects all columns from table Book, ordering by Title, in ascending order. To specify
|
|
descending order, add DESC after the ORDER BY Title clause.
|
|
|
|
{\tt SELECT Title FROM Book WHERE RetailPriceAmount >= 20.0 AND RetailPriceAmount <= 35.0}
|
|
|
|
Selects records where RetailPriceAmount conforms to the WHERE expression.
|
|
|
|
\subsubsection{Update}\label{odbcupdateexample}
|
|
|
|
Updates records in a table.
|
|
|
|
Example:
|
|
|
|
{\tt UPDATE Incident SET X = 123 WHERE ASSET = 'BD34'}
|
|
|
|
This example sets a field in column `X' to the number 123, for the record
|
|
where the column ASSET has the value `BD34'.
|
|
|