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The SQL Module uses driver plugins in order to communicate with different database APIs. Since the SQL Module API is database-independent, all database-specific code is contained within these drivers. Several drivers are supplied with Qt and other drivers can be added. The driver source code is supplied and can be used as a model for writing your own drivers.
To build a driver plugin you need the client API that is shipped with every Database Management System (DBMS). Most installation programs also allow you to install "development libraries", and these are what you need. These libraries are responsible for the low-level communication with the DBMS.
The currently available drivers shipped with Qt are:
Note that not all of the plugins are shipped with the Qt Free Edition due to license incompatibilities with the GPL.
The Qt configure script automatically detects the available client libraries on your machine. Run "configure -help" to see what drivers may be built. You should get an output similar to this:
Possible values for <driver>: [ mysql oci odbc psql tds ] Auto-Detected on this system: [ mysql psql ]
Note that on Windows, the configure script doesn't currently do any auto-detection.
Note that configure cannot detect the neccessary libraries and include files if they are not in the standard paths, so it may be necessary to specify these paths using the "-I" and "-L" switches. If your MySQL include files are installed in /usr/local/mysql (or in C:\mysql\include on Windows), then pass the following parameter to configure: "-I/usr/local/mysql" (or "-I C:\mysql\include" for Windows).
Note that on Windows the parameter -I doesn't allow spaces in filenames, so use the 8.3 name instead, i.e. use "C:\progra~1\mysql" instead of "C:\program files\mysql".
Use the -qt-sql-<driver> parameter to build the database driver statically into your Qt library or -plugin-sql-<driver> to build the driver as a plugin. Look at the chapters below for additional information about required libraries.
MySQL 3.x doesn't support SQL transactions by default. There are some backends which offer this functionality. Recent versions of the MySQL client libraries (>3.23.34) allow you to use transactions on those modified servers.
If you have a recent client library and connect to a transaction-enabled MySQL server, a call to the QSqlDriver::hasFeature( QSqlDriver::Transactions ) function returns TRUE and SQL transactions can be used.
You can find information about MySQL on http://www.mysql.com
You need the MySQL header files and as well as the shared library "libmysqlclient.so". Depending on your Linux distribution you need to install a package which is usually called "mysql-devel".
Tell qmake where to find the MySQL header files and shared libraries (here it is assumed that MySQL is installed in /usr/local) and run make:
cd $QTDIR/plugins/src/sqldrivers/mysql qmake -o Makefile "INCLUDEPATH+=/usr/local/include" "LIBS+=-L/usr/local/lib -lmysqlclient" mysql.pro make
You need to get the MySQL installation files. Run SETUP.EXE and choose "Custom Install". Install the "Libs & Include Files" Module. Build the plugin as follows (here it is assumed that MySQL is installed in C:\MYSQL):
cd %QTDIR%\plugins\src\sqldrivers\mysql qmake -o Makefile "INCLUDEPATH+=C:\MYSQL\INCLUDE" "LIBS+=C:\MYSQL\LIB\OPT\LIBMYSQL.LIB" mysql.pro nmake
If you are not using a Microsoft compiler, replace "nmake" with "make" in the statement above.
The Qt OCI plugin supports both Oracle 8 and Oracle 9. After connecting to the Oracle server, the plugin will auto-detect the database version and enable features accordingly.
If the Oracle server supports Unicode, the OCI plugin will use UTF-8 encoding to communicate with the server. When accessing an Oracle 8 database, only retrieval of Unicode strings are supported. For Oracle 9 both retrieval and insertion of Unicode strings are supported.
Binary Large Objects (BLOBs) can be retrieved, but be aware that this process may require a lot of memory. We recommend using separate queries and retrieving only one BLOB at a time.
Inserting BLOB fields is limited to Oracle's maximum SQL string size.
All files required to build driver should ship with the standard Oracle Client install.
Oracle library files required to build driver:
Tell qmake where to find the Oracle header files and shared libraries (it is assumed that the variable $ORACLE_HOME points to the directory where Oracle is installed) and run make:
If you are using Oracle 8:
cd $QTDIR/plugins/src/sqldrivers/oci qmake -o Makefile "INCLUDEPATH+=$ORACLE_HOME/rdbms/public $ORACLE_HOME/rdbms/demo" "LIBS+=-L$ORACLE_HOME/lib -lclntsh -lwtc8" oci.pro make
For Oracle version 9:
cd $QTDIR/plugins/src/sqldrivers/oci qmake -o Makefile "INCLUDEPATH+=$ORACLE_HOME/rdbms/public $ORACLE_HOME/rdbms/demo" "LIBS+=-L$ORACLE_HOME/lib -lclntsh -lwtc9" oci.pro make
Choosing the option "Programmer" in the Oracle Client Installer from the Oracle Client Installation CD is sufficient to build the plugin.
Build the plugin as follows (here it is assumed that Oracle Client is installed in C:\oracle):
set INCLUDE=%INCLUDE%;c:\oracle\oci\include set LIB=%LIB%;c:\oracle\oci\lib\msvc cd %QTDIR%\plugins\src\sqldrivers\oci qmake -o Makefile oci.pro nmake
When you run your application you will also need to add the oci.dll path to your PATH environment variable:
set PATH=%PATH%;c:\oracle\bin
If you are not using a Microsoft compiler, replace "nmake" with "make" in the statement above.
ODBC is a general interface that allows you to connect to multiple DBMS using a common interface. The QODBC3 driver allows you to connect to an ODBC driver manager and access the available data sources. Note that you also need to install and configure ODBC drivers for the ODBC driver manager that is installed on your system. The QODBC3 plugin then allows you to use these data sources in your Qt project.
On Windows systems after 95 an ODBC driver manager should be installed by default, for Unix systems there are some implementations which have to be installed first. Note that every client that uses your application is required to have an ODBC driver manager installed, otherwise the QODBC3 plugin will not work.
Be aware that when connecting to an ODBC datasource you must pass in the name of the ODBC datasource to the QSqlDatabase::setDatabaseName() function - not the actual database name.
The QODBC3 Plugin needs an ODBC compliant driver manager version 2.0 or greater to work. Some ODBC drivers claim to be version 2.0 compliant, but do not offer all needed functionality. The QODBC3 plugin therefore checks whether the data source can be used after a connection has been established and refuses to work if the check fails. If you don't like this behaviour, you can remove the #define ODBC_CHECK_DRIVER line from the file qsql_odbc.cpp. Do this at your own risk!
Note that some DBMSs, like Microsoft Access, does not return the primary index for a table through ODBC. You can still use the QODBC3 driver, but you will have to manage table indices yourself (through QSqlCursor::setPrimaryIndex()), otherwise INSERT, UPDATE and DELETE operations will fail in data-aware Qt widgets. You will have to comment out the #define ODBC_CHECK_DRIVER line and recompile the driver, otherwise you will not be able to connect to a Microsoft Access database.
It is recommended that you use unixODBC. You can find the newest version and ODBC drivers at http://www.unixodbc.org. You need the unixODBC header files and shared libraries.
Tell qmake where to find the unixODBC header files and shared libraries (here it is assumed that unixODBC is installed in /usr/local/unixODBC) and run make:
cd $QTDIR/plugins/src/sqldrivers/odbc qmake "INCLUDEPATH+=/usr/local/unixODBC/include" "LIBS+=-L/usr/local/unixODBC/lib -lodbc" make
The ODBC header and include files should already be installed in the right directories. You just have to build the plugin as follows:
cd %QTDIR%\plugins\src\sqldrivers\odbc qmake -o Makefile odbc.pro nmake
If you are not using a Microsoft compiler, replace "nmake" with "make" in the statement above.
The QPSQL7 driver supports both version 6 and 7 of PostgreSQL. We recommend compiling the plugin with a recent version of the PostgreSQL Client API (libpq) because it is more stable and still downward compatible.
If you want to link the plugin against the libpq shipped with version 6 we recomment a recent version like PostgreSQL 6.5.3, otherwise a connection to a version 7 server may not work.
The driver auto-detects the server version of PostgreSQL after a connection was successful. If the server is too old or the version information cannot be determined a warning is issued.
For more information about PostgreSQL visit http://www.postgresql.org.
The QPSQL7 driver automatically detects whether the PostgreSQL database you are connecting to supports Unicode or not. Unicode is automatically used if the server supports it. Note that the driver only supports the UTF-8 encoding. If your database uses any other encoding, the server has to be compiled with Unicode conversion support.
Unicode support was introduced in PostgreSQL version 7.1 and it will only work if both the server and the client library have been compiled with multibyte support. More information about how to set up a multibyte enabled PostgreSQL server can be found in the PostgreSQL Administrator Guide, Chapter 5.
Binary Large Objects can be fetched but not inserted. Please use the PostgreSQL command lo_import to insert binary data into the database.
Just installing "libpq.so" and the corresponding header files is unfortunately not sufficient. You have to get the whole source distribution and run the configure script once (there is no need to build it if you have already installed a binary distribution).
Tell qmake where to find the PostgreSQL header files and shared libraries (here it is assumed that you extracted the PostgreSQL source code in /usr/src/psql and the shared library is installed in /usr/lib) and run make:
cd $QTDIR/plugins/src/sqldrivers/psql qmake -o Makefile "INCLUDEPATH+=/usr/src/psql/src/include /usr/src/psql/src/interfaces/libpq" "LIBS+=-L/usr/lib -lpq" psql.pro make
Under Unix, two libraries are available which support the TDS protocol:
- FreeTDS, a free implementation of the TDS protocol ( http://www.freetds.org ). Note that FreeTDS is not yet stable, so some functionality may not work as expected.
- Sybase Open Client, available from http://www.sybase.com Note for Linux users: Get the Open Client RPM from http://linux.sybase.com
Regardless of which library you use, the shared object file "libsybdb.so" is needed. Set the SYBASE environment variable to point to the directory where you installed the client library and execute qmake:
cd $QTDIR/plugins/src/sqldrivers/tds qmake -o Makefile "INCLUDEPATH=$SYBASE/include" "LIBS=-L$SYBASE/lib -lsybdb" make
You can either use the DB-Library supplied by Microsoft or the Sybase Open Client ( http://www.sybase.com ). You have to include NTWDBLIB.LIB to build the plugin:
cd %QTDIR%\plugins\src\sqldrivers\tds qmake -o Makefile "LIBS+=NTWDBLIB.LIB" tds.pro nmake
By default the Microsoft library is used on Windows, if you want to force the use of the Sybase Open Client, you have to define Q_USE_SYBASE in %QTDIR%\src\sql\drivers\tds\qsql_tds.cpp.
You should always use client libraries that have been compiled with the same compiler as you are using for your project. If you cannot get a source distibution to compile the client libraries yourself, you have to make sure that the pre-compiled library is compatible with your compiler, otherwise you will get a lot of "undefined symbols" errors. Some compilers have tools to convert libraries, e.g. Borland ships the tool COFF2OMF.EXE to convert libraries that have been generated with Microsoft Visual C++.
If the compilation of a plugin succeeds but it cannot be loaded, make sure that the following requirements are met:
QSqlDatabase is responsible for loading and managing database driver plugins. When a database is added (see QSqlDatabase::addDatabase()), the appropriate driver plugin is loaded (using QSqlDriverPlugin). QSqlDatabase relies on the driver plugin to provide interfaces for QSqlDriver and QSqlResult.
QSqlDriver is an abstract base class which defines the functionality of a SQL database driver. This includes functions such as QSqlDriver::open() and QSqlDriver::close(). QSqlDriver is responsible for connecting to a database, establish the proper environment, etc. In addition, QSqlDriver can create QSqlQuery objects appropriate for the particular database API. QSqlDatabase forwards many of its function calls directly to QSqlDriver which provides the concrete implementation.
QSqlResult is an abstract base class which defines the functionality of a SQL database query. This includes statements such as SELECT, UPDATE, or ALTER TABLE. QSqlResult contains functions such as QSqlResult::next() and QSqlResult::value(). QSqlResult is responsible for sending queries to the database, returning result data, etc. QSqlQuery forwards many of its function calls directly to QSqlResult which provides the concrete implementation.
QSqlDriver and QSqlResult are closely connected. When implementing a Qt SQL driver, both of these classes must to be subclassed and the abstract virtual methods in each class must be implemented.
To implement a Qt SQL driver as a plugin (so that it is recognized and loaded by the Qt library at runtime), the driver must use the Q_EXPORT_PLUGIN macro. Please read the Qt Plugin documentation for more information on this. You can also check out how this is done in the SQL plugins that is provided with Qt in QTDIR/plugins/src/sqldrivers and QTDIR/src/sql/drivers.
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