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# SQL Module - Drivers

- Introduction
- Building the drivers using configure
- Building the plugins manually
  - QMYSQL3 - MySQL 3.x and MySQL 4.x
    - General information
    - How to build the plugin on Unix/Linux
    - How to build the plugin on Windows
  - QOCI8 - Oracle Call Interface (OCI)
    - General information
    - Unicode support
    - BLOB/LOB support
    - How to build the plugin on Unix/Linux
    - How to build the plugin on Windows
  - QODBC3 - Open Database Connectivity (ODBC)
    - General information
    - Unicode support
    - How to build the plugin on Unix/Linux
    - How to build the plugin on Windows
  - QPSQL7 - PostgreSQL version 6 and 7
    - General information
    - Unicode support
    - BLOB support
    - How to build the plugin on Unix/Linux
    - How to build the plugin on Windows
  - QTDS7 - Sybase Adaptive Server and Microsoft SQL Server
    - How to build the plugin on Unix/Linux
    - How to build the plugin on Windows
- Troubleshooting
- How to write your own database driver

## Introduction

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 your 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 drivers shipped with Qt are:

- QMYSQL3 - MySQL Driver
- QOCI8 - Oracle Call Interface Driver, version 8 and 9
- QODBC3 - Open Database Connectivity Driver
- QPSQL7 - PostgreSQL v6.x and v7.x Driver
- QTDS7 - Sybase Adaptive Server and Microsoft SQL Server Driver

Note that not all of the plugins are shipped with the Qt Free Edition due to license incompatibilities with the GPL.

## Building the drivers using configure

The Qt configure script automatically detects the available client libraries on your machine. Run "configure -help" to see what drivers can 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 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. For example, if your MySQL include files are installed in `/usr/local/mysql` (or in `C:\mysql\include` configure: `-I/usr/local/mysql` (or `-I C:\mysql\include` Windows).

Note that on Windows the -I parameter 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 sections that follow for additional information about required libraries.

## Building the plugins manually

### QMYSQL3 - MySQL 3.x and MySQL 4.x

#### General information

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.

If the plugin is compiled against MySQL 4.x client libraries, transactions are enabled by default.

You can find information about MySQL on http://www.mysql.com

#### How to build the plugin on Unix/Linux

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

#### How to build the plugin on Windows

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.

### QOCI8 - Oracle Call Interface (OCI)

#### General information

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.

#### Unicode support

If the Oracle server supports Unicode, the OCI plugin will use UTF-8 encoding to communicate with the server.

#### BLOB/LOB support

Binary Large Objects (BLOBs) can be read and written, but be aware that this process may require a lot of memory.

Note that Oracle 9 doesn't support scrollable result sets with LOB columns, you have to use a forward only query to select LOB fields (see QSqlQuery::setForwardOnly()).

Inserting BLOBs should be done using either a prepared query where the BLOBs are bound to placeholders, or QSqlCursor which uses a prepared query to do this internally (see $QTDIR/examples/sql/blob).

#### How to build the plugin on Unix/Linux

All files required to build driver should ship with the standard Oracle Client install.

Oracle library files required to build driver:

- `libclntsh.so`
- `libclntsh.so.8.0` (Oracle 8) or `libclntsh.so.9.0` (Oracle 9)
- `libwtc8.so` or `libwtc9.so`

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

Note that some versions of the OCI 8 client libraries contain a bug that makes programs linked to these libraries segfault on exit. This only happens if the QOCI8 driver is compiled as a plugin. To work around this problem, either compile the driver into the Qt libray itself, or configure Qt with the option '-DQT_NO_LIBRARY_UNLOAD'.

#### How to build the plugin on Windows

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.

### QODBC3 - Open Database Connectivity (ODBC)

#### General information

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 must 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 later to work. Some ODBC drivers claim to be version 2.0 compliant, but do not offer all the necessary 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!

If you experience very slow access of the ODBC datasource, make sure that ODBC call tracing is turned off in the ODBC datasource manager.

#### Unicode support

The QODBC3 Plugin will use the Unicode API if UNICODE is defined. On Windows NT based systems, this is the default. Note that the ODBC driver and the DBMS have to support Unicode as well.

For the Oracle 9 ODBC driver (Windows), it is neccessary to check "SQL_WCHAR support" in the ODBC driver manager otherwise Oracle will convert all Unicode strings to local 8 bit.

#### How to build the plugin on Unix/Linux

It is recommended that you use unixODBC. You can find the latest 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

#### How to build the plugin on Windows

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.

### QPSQL7 - PostgreSQL version 6 and 7

#### General information

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 recommend 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.

#### Unicode support

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 must 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.

#### BLOB support

Binary Large Objects can be fetched but not inserted. Use the PostgreSQL command `lo_import` to insert binary data into the database.

#### How to build the plugin on Unix/Linux

Just installing the pq client library and the corresponding header files is not sufficient. You have to get the PostgreSQL source distribution and run the configure script. If you've already installed a binary distribution you don't need to build it. The source distribution is needed because the QPSQL7 plugin relies on a couple of header files that are usually not a part of the binary distribution.

To make `qmake` find the PostgreSQL header files and shared libraries, run `qmake` the following way (assuming that the PostgreSQL sources can be found in `/usr/src/psql`):

    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

#### How to build the plugin on Windows

Unpack and build the PostgreSQL source distribution as described in the PostgreSQL documentation. Assuming the PostgreSQL sources resides in `C:\psql`, build the plugin as follows:

    cd %QTDIR%\plugins\src\sqldrivers\psql
    qmake -o Makefile "INCLUDEPATH+=C:\psql\src\include C:\psql\src\interfaces\libpq" psql.pro
    nmake

Remember to add the path to the `libpq.dll` library to your PATH environment variable so that Windows can find it. In this case that would be `C:\psql\src\interfaces\libpq\Release`. If you are not using a Microsoft compiler, replace `nmake` with `make` in the statement above.

### QTDS7 - Sybase Adaptive Server and Microsoft SQL Server

#### How to build the plugin on Unix/Linux

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

#### How to build the plugin on Windows

You can either use the DB-Library supplied by Microsoft or the Sybase Open Client (http://www.sybase.com). You must 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 must define `Q_USE_SYBASE` in `%QTDIR%\src\sql\drivers\tds\qsql_tds.cpp`.

## Troubleshooting

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 must 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:

- Ensure that you are using a shared Qt library; you cannot use the plugins with a static build.
- Ensure that the environment variable `QTDIR` points to the right directory. Go to the `$QTDIR/plugins/sqldrivers` directory and make sure that the plugin exists in that directory.
- Ensure that the client libraries of the DBMS are available on the system. On Unix, run the command `ldd` and pass the name of the plugin as parameter, for example `ldd libqsqlmysql.so`. You will get a warning if any of the client libraries couldn't be found. On Windows, you can use the dependency walker of Visual Studio.

## How to write your own database driver

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`, and `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. 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`.

The following code can be used as a skeleton for a SQL driver:

    class QNullResult : public QSqlResult
    {
    public:
        QNullResult( const QSqlDriver* d ): QSqlResult( d ) {}
        ~QNullResult() {}
    protected:
        QVariant    data( int ) { return QVariant(); }
        bool        reset ( const QString& ) { return FALSE; }
        bool        fetch( int ) { return FALSE; }
        bool        fetchFirst() { return FALSE; }
        bool        fetchLast() { return FALSE; }
        bool        isNull( int ) { return FALSE; }
        QSqlRecord  record() { return QSqlRecord(); }
        int         size()  { return 0; }
        int         numRowsAffected() { return 0; }
    };

    class QNullDriver : public QSqlDriver
    {
    public:
        QNullDriver(): QSqlDriver() {}
        ~QNullDriver() {}
        bool    hasFeature( DriverFeature ) const { return FALSE; }
        bool    open( const QString&,
                      const QString&,
                      const QString&,
                      const QString&,
                      int ) { return FALSE; }
        void    close() {}
        QSqlQuery createQuery() const { return QSqlQuery( new QNullResult( this ) ); }
    };