path: root/doc/src/sql-programming/sql-driver.qdoc

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/*!
    \page sql-driver.html
    \title SQL Database Drivers
    \brief How to configure and install QtSql drivers for supported databases.

    \ingroup best-practices

    The QtSql module uses driver \l{How to Create Qt
    Plugins}{plugins} to communicate with the different database
    APIs. Since Qt's 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
    \l{#development}{writing your own drivers}.

    \tableofcontents

    \section1 Supported Databases

    The table below lists the drivers included with Qt. Due to
    license incompatibilities with the GPL, not all of the plugins
    are provided with Open Source Versions of Qt.

    \table
    \header \o Driver name \o DBMS
    \row \o \link #QDB2 QDB2\endlink \o IBM DB2 (version 7.1 and above)
    \row \o \link #QIBASE QIBASE\endlink \o Borland InterBase
    \row \o \link #QMYSQL QMYSQL\endlink \o MySQL
    \row \o \link #QOCI QOCI\endlink \o Oracle Call Interface Driver
    \row \o \link #QODBC QODBC\endlink
         \o Open Database Connectivity (ODBC) - Microsoft SQL Server and other
            ODBC-compliant databases
    \row \o \link #QPSQL QPSQL\endlink \o PostgreSQL (versions 7.3 and above)
    \row \o \link #QSQLITE2 QSQLITE2\endlink \o SQLite version 2
    \row \o \link #QSQLITE QSQLITE\endlink \o SQLite version 3
    \row \o \link #QTDS QTDS\endlink \o Sybase Adaptive Server
    \endtable

    \bold{Note:} To build a driver plugin you need to have the appropriate
    client library for your Database Management System (DBMS). This provides
    access to the API exposed by the DBMS, and is typically shipped with it.
    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.

    \target building
    \section1 Building the Drivers Using Configure

    On Unix and Mac OS X, the Qt \c configure script tries to
    automatically detect the available client libraries on your
    machine. Run \c{configure -help} to see what drivers can be
    built. You should get an output similar to this:

    \snippet doc/src/snippets/code/doc_src_sql-driver.qdoc 0

    The \c configure script 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 \c -I and \c -L
    command-line options. For example, if your MySQL include files
    are installed in \c /usr/local/mysql (or in \c{C:\mysql\include}
    on Windows), then pass the following parameter to configure: \c
    -I/usr/local/mysql (or \c{-I C:\mysql\include} for Windows).

    On Windows the \c -I parameter doesn't accept spaces in
    filenames, so use the 8.3 name instead; for example, use
    \c{C:\progra~1\mysql} instead of \c{C:\Program Files\mysql}.

    Use the \c{-qt-sql-<driver>} parameter to build the database driver
    statically into your Qt library or \c{-plugin-sql-<driver>} to build
    the driver as a plugin. Look at the sections that follow for
    additional information about required libraries.

    \target buildingmanually
    \section1 Building the Plugins Manually

    \target QMYSQL
    \section2 QMYSQL for MySQL 4 and higher

    \section3 QMYSQL Stored Procedure Support

    MySQL 5 introduces stored procedure support at the SQL level, but no
    API to control IN, OUT and INOUT parameters. Therefore, parameters
    have to be set and read using SQL commands instead of QSqlQuery::bindValue().

    Example stored procedure:

    \snippet doc/src/snippets/code/doc_src_sql-driver.qdoc 1

    Source code to access the OUT values:

    \snippet doc/src/snippets/code/doc_src_sql-driver.qdoc 2

    \bold{Note:} \c{@outval1} and \c{@outval2} are variables local to the current
    connection and will not be affected by queries sent from another host
    or connection.

    \section3 Embedded MySQL Server

    The MySQL embedded server is a drop-in replacement for the normal
    client library. With the embedded MySQL server, a MySQL server is
    not required to use MySQL functionality.

    To use the embedded MySQL server, simply link the Qt plugin to \c
    libmysqld instead of libmysqlclient. This can be done by replacing
    \c -lmysqlclient_r by \c -lmysqld in the \c qmake command in the
    section below.

    Please refer to the MySQL documentation, chapter "libmysqld, the Embedded
    MySQL Server Library" for more information about the MySQL embedded server.

    \section3 How to Build the QMYSQL Plugin on Unix and Mac OS X

    You need the MySQL header files and as well as the shared library
    \c{libmysqlclient.so}. Depending on your Linux distribution you may
    need to install a package which is usually called "mysql-devel".

    Tell \l qmake where to find the MySQL header files and shared
    libraries (here it is assumed that MySQL is installed in
    \c{/usr/local}) and run \c{make}:

    \snippet doc/src/snippets/code/doc_src_sql-driver.qdoc 3

    After installing Qt, as described in the \l{Installing Qt on X11
    Platforms} document, you also need to install the plugin in the
    standard location:

    \snippet doc/src/snippets/code/doc_src_sql-driver.qdoc 4

    \section3 How to Build the QMYSQL Plugin on Windows

    You need to get the MySQL installation files. Run \c 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{C:\MySQL}):

    \snippet doc/src/snippets/code/doc_src_sql-driver.qdoc 5

    If you are not using a Microsoft compiler, replace \c nmake with \c
    make in the line above.

    \note This database plugin is not supported for Windows CE.

    \note Including \c{"-o Makefile"} as an argument to \l qmake to
    tell it where to build the makefile can cause the plugin to be
    built in release mode only. If you are expecting a debug version
    to be built as well, don't use the \c{"-o Makefile"} option.

    \target QOCI
    \section2 QOCI for the Oracle Call Interface (OCI)

    \section3 General Information about the OCI plugin

    The Qt OCI plugin supports Oracle 9i, 10g and higher. After
    connecting to the Oracle server, the plugin will auto-detect the
    database version and enable features accordingly.

    It's possible to connect to a Oracle database without a tnsnames.ora file.
    This requires that the database SID is passed to the driver as the database
    name and that a hostname is given.

    \section3 OCI User Authentication

    The Qt OCI plugin supports authentication using
    external credentials (OCI_CRED_EXT). Usually, this means that the database
    server will use the user authentication provided by the operating system
    instead of its own authentication mechanism.

    Leave the username and password empty when opening a connection with
    QSqlDatabase to use the external credentials authentication.

    \section3 OCI BLOB/LOB Support

    Binary Large Objects (BLOBs) can be read and written, but be aware
    that this process may require a lot of memory. You should 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 QSqlTableModel, which uses a prepared
    query to do this internally.

    \section3 How to Build the OCI Plugin on Unix and Mac OS X

    For Oracle 10g, all you need is the "Instant Client Package - Basic" and
    "Instant Client Package - SDK". For Oracle prior to 10g, you require
    the standard Oracle client and the SDK packages.

    Oracle library files required to build the driver:

    \list
    \i \c libclntsh.so (all versions)
    \i \c libwtc9.so (only Oracle 9)
    \endlist

    Tell \c qmake where to find the Oracle header files and shared
    libraries and run make:

    For Oracle version 9:
    \snippet doc/src/snippets/code/doc_src_sql-driver.qdoc 6

    For Oracle version 10, we assume that you installed the RPM packages of the
    Instant Client Package SDK (you need to adjust the version number accordingly):
    \snippet doc/src/snippets/code/doc_src_sql-driver.qdoc 7

    \bold{Note:} If you are using the Oracle Instant Client package,
    you will need to set LD_LIBRARY_PATH when building the OCI SQL plugin
    and when running an applicaiton that uses the OCI SQL plugin. You can
    avoid this requirement by setting and RPATH and listing all of the
    libraries to link to. Here is an example:
    \snippet doc/src/snippets/code/doc_src_sql-driver.qdoc 32

    If you wish to build the OCI plugin manually with this method the procedure looks like this:
    \snippet doc/src/snippets/code/doc_src_sql-driver.qdoc 33

    \section3 How to Build the OCI 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{C:\oracle}):

    \snippet doc/src/snippets/code/doc_src_sql-driver.qdoc 8

    If you are not using a Microsoft compiler, replace \c nmake with \c
    make in the line above.

    When you run your application you will also need to add the \c oci.dll
    path to your \c PATH environment variable:

    \snippet doc/src/snippets/code/doc_src_sql-driver.qdoc 9

    \bold{Note:} This database plugin is not supported for Windows CE.

    \target QODBC
    \section2 QODBC for Open Database Connectivity (ODBC)

    \section3 General Information about the ODBC plugin

    ODBC is a general interface that allows you to connect to multiple
    DBMSs using a common interface. The QODBC 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 QODBC plugin
    then allows you to use these data sources in your Qt applications.

    \bold{Note:} You should use native drivers in preference to the ODBC
    driver where they are available. ODBC support can be used as a fallback
    for compliant databases if no native drivers are available.

    On Windows 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
    QODBC 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 rather than the actual database name.

    The QODBC 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 QODBC 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 behavior, you can remove the \c{#define
    ODBC_CHECK_DRIVER} line from the file \c{qsql_odbc.cpp}. Do this at
    your own risk!

    By default, Qt instructs the ODBC driver to behave as an ODBC 2.x
    driver. However, for some \e{driver-manager/ODBC 3.x-driver}
    combinations (e.g., \e{unixODBC/MaxDB ODBC}), telling the ODBC
    driver to behave as a 2.x driver can cause the driver plugin to
    have unexpected behavior. To avoid this problem, instruct the ODBC
    driver to behave as a 3.x driver by
    \l{QSqlDatabase::setConnectOptions()} {setting the connect option}
    \c{"SQL_ATTR_ODBC_VERSION=SQL_OV_ODBC3"} before you
    \l{QSqlDatabase::open()} {open your database connection}. Note
    that this will affect multiple aspects of ODBC driver behavior,
    e.g., the SQLSTATEs.  Before setting this connect option, consult
    your ODBC documentation about behavior differences you can expect.

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

    Some drivers don't support scrollable cursors. In that case case only
    queries in forwardOnly mode can be used successfully.

    \section3 ODBC Stored Procedure Support

    With Microsoft SQL Server the result set returned by a stored
    procedure that uses the return statement, or returns multiple result
    sets, will be accessible only if you set the query's forward only
    mode to \e forward using \l QSqlQuery::setForwardOnly().

    \snippet doc/src/snippets/code/doc_src_sql-driver.qdoc 10

    \bold{Note:} The value returned by the stored procedure's return statement
    is discarded.

    \section3 ODBC Unicode Support

    The QODBC 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 must also support Unicode.

    Some driver managers and drivers don't support UNICODE. To use the
    QODBC plugin with such drivers it has to be compiled with the
    Q_ODBC_VERSION_2 defined.

    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.

    \section3 How to Build the ODBC Plugin on Unix and Mac OS X

    It is recommended that you use unixODBC. You can find the latest
    version and ODBC drivers at \l http://www.unixodbc.org.
    You need the unixODBC header files and shared libraries.

    Tell \c qmake where to find the unixODBC header files and shared
    libraries (here it is assumed that unixODBC is installed in
    \c{/usr/local/unixODBC}) and run \c{make}:

    \snippet doc/src/snippets/code/doc_src_sql-driver.qdoc 11

    \section3 How to Build the ODBC 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:

    \snippet doc/src/snippets/code/doc_src_sql-driver.qdoc 12

    If you are not using a Microsoft compiler, replace \c nmake with \c
    make in the line above.

    \bold{Note:} This database plugin is not officially supported for Windows CE.

    \target QPSQL
    \section2 QPSQL for PostgreSQL (Version 7.3 and Above)

    \section3 General Information about the QPSQL driver

    The QPSQL driver supports version 7.3 and higher of the PostgreSQL server.
    We recommend that you use a client library from version 7.3.15, 7.4.13,
    8.0.8, 8.1.4 or more recent as these versions contain security fixes, and
    as the QPSQL driver might not build with older versions of the client
    library depending on your platform.

    For more information about PostgreSQL visit \l http://www.postgresql.org.

    \section3 QPSQL Unicode Support

    The QPSQL 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.

    \section3 QPSQL BLOB Support

    Binary Large Objects are supported through the \c BYTEA field type in
    PostgreSQL server versions >= 7.1.

    \section3 How to Build the QPSQL Plugin on Unix and Mac OS X

    You need the PostgreSQL client library and headers installed.

    To make \c qmake find the PostgreSQL header files and shared
    libraries, run \c qmake the following way (assuming that the
    PostgreSQL client is installed in \c{/usr}):

    \snippet doc/src/snippets/code/doc_src_sql-driver.qdoc 13

    After installing Qt, as described in the \l{Installing Qt on X11 Platforms} document,
    you also need to install the plugin in the standard location:

    \snippet doc/src/snippets/code/doc_src_sql-driver.qdoc 14

    \section3 How to Build the QPSQL Plugin on Windows

    Install the appropriate PostgreSQL developer libraries for your
    compiler. Assuming that PostgreSQL was installed in \c{C:\psql},
    build the plugin as follows:

    \snippet doc/src/snippets/code/doc_src_sql-driver.qdoc 15

    Users of MinGW may wish to consult the following online document:
    \l{Compiling PostgreSQL On Native Win32 FAQ}.

    \bold{Note:} This database plugin is not supported for Windows CE.

    \target QTDS
    \section2 QTDS for Sybase Adaptive Server
    \section3 General Information about QTDS

    It is not possible to set the port with QSqlDatabase::setPort() due to limitations in the
    Sybase client library. Refer to the Sybase documentation for information on how to set up
    a Sybase client configuration file to enable connections to databases on non-default ports.

    \section3 How to Build the QDTS Plugin on Unix and Mac OS X

    Under Unix, two libraries are available which support the TDS protocol:

    \list
    \i FreeTDS, a free implementation of the TDS protocol
      (\l{http://www.freetds.org}). Note that FreeTDS is not yet stable,
      so some functionality may not work as expected.

    \i Sybase Open Client, available from \l{http://www.sybase.com}.
      Note for Linux users: Get the Open Client RPM from
      \l{http://linux.sybase.com}.
    \endlist

    Regardless of which library you use, the shared object file
    \c{libsybdb.so} is needed. Set the \c SYBASE environment variable to
    point to the directory where you installed the client library and
    execute \c{qmake}:

    \snippet doc/src/snippets/code/doc_src_sql-driver.qdoc 16

    \section3 How to Build the QDTS Plugin on Windows

    You can either use the DB-Library supplied by Microsoft or the Sybase
    Open Client (\l{http://www.sybase.com}). You must include \c
    NTWDBLIB.LIB to build the plugin:

    \snippet doc/src/snippets/code/doc_src_sql-driver.qdoc 17

    By default the Microsoft library is used on Windows, if you want to
    force the use of the Sybase Open Client, you must define \c
    Q_USE_SYBASE in \c{%QTDIR%\src\sql\drivers\tds\qsql_tds.cpp}. If you
    are not using a Microsoft compiler, replace \c nmake with \c make in
    the line above.

    \bold{Note:} This database plugin is not supported for Windows CE.

    \target QDB2
    \section2 QDB2 for IBM DB2 (Version 7.1 and Above)

    \section3 General Information about QDB2

    The Qt DB2 plugin makes it possible to access IBM DB2 databases. It
    has been tested with IBM DB2 v7.1 and 7.2. You must install the IBM
    DB2 development client library, which contains the header and library
    files necessary for compiling the QDB2 plugin.

    The QDB2 driver supports prepared queries, reading/writing of Unicode
    strings and reading/writing of BLOBs.

    We suggest using a forward-only query when calling stored procedures
    in DB2 (see QSqlQuery::setForwardOnly()).

    \section3 How to Build the QDB2 Plugin on Unix and Mac OS X

    \snippet doc/src/snippets/code/doc_src_sql-driver.qdoc 18

    After installing Qt, as described in the \l{Installing Qt on X11 Platforms} document,
    you also need to install the plugin in the standard location:

    \snippet doc/src/snippets/code/doc_src_sql-driver.qdoc 19

    \section3 How to Build the QDB2 Plugin on Windows

    The DB2 header and include files should already be installed in the
    right directories. You just have to build the plugin as follows:

    \snippet doc/src/snippets/code/doc_src_sql-driver.qdoc 20

    If you are not using a Microsoft compiler, replace \c nmake
    with \c make in the line above.

    \bold{Note:} This database plugin is not supported for Windows CE.

    \target QSQLITE2
    \section2 QSQLITE2 for SQLite Version 2

    The Qt SQLite 2 plugin is offered for compatibility. Whenever
    possible, use the \l{#QSQLITE}{version 3 plugin} instead. The
    build instructions for version 3 apply to version 2 as well.

    \target QSQLITE
    \section2 QSQLITE for SQLite (Version 3 and Above)

    \section3 General Information about QSQLITE

    The Qt SQLite plugin makes it possible to access SQLite
    databases. SQLite is an in-process database, which means that it
    is not necessary to have a database server. SQLite operates on a
    single file, which must be set as the database name when opening
    a connection. If the file does not exist, SQLite will try to
    create it. SQLite also supports in-memory databases, simply pass
    ":memory:" as the database name.

    SQLite has some restrictions regarding multiple users and
    multiple transactions. If you try to read/write on a resource from different
    transactions, your application might freeze until one transaction commits
    or rolls back. The Qt SQLite driver will retry to write to a locked resource
    until it runs into a timeout (see \c{QSQLITE_BUSY_TIMEOUT}
    at QSqlDatabase::setConnectOptions()).

    In SQLite any column, with the exception of an INTEGER PRIMARY KEY column,
    may be used to store any type of value. For instance, a column declared as
    INTEGER may contain an integer value in one row and a text value in the
    next. This is due to SQLite associating the type of a value with the value
    itself rather than with the column it is stored in. A consequence of this
    is that the type returned by QSqlField::type() only indicates the field's
    recommended type. No assumption of the actual type should be made from
    this and the type of the individual values should be checked.

    The driver is locked for updates while a select is executed. This
    may cause problems when using QSqlTableModel because Qt's item views
    fetch data as needed (with QSqlQuery::fetchMore() in the case of
    QSqlTableModel).

    You can find information about SQLite on \l{http://www.sqlite.org}.

    \section3 How to Build the QSQLITE Plugin

    SQLite version 3 is included as a third-party library within Qt.
    It can be built by passing the following parameters to the
    configure script: \c{-plugin-sql-sqlite} (build as a plugin) or
    \c{-qt-sql-sqlite} (linked directly into the Qt library).

    If you don't want to use the SQLite library included with Qt, you
    can build it manually (replace \c $SQLITE by the directory where
    SQLite resides):

    \snippet doc/src/snippets/code/doc_src_sql-driver.qdoc 21

    After installing Qt, as described in the \l{Installing Qt on X11 Platforms} document,
    you also need to install the plugin in the standard location:

    \snippet doc/src/snippets/code/doc_src_sql-driver.qdoc 22

    On Windows:

    \snippet doc/src/snippets/code/doc_src_sql-driver.qdoc 23

    \section3 QSQLITE File Format Compatibility

    SQLite minor releases sometimes break file format forward compatibility.
    For example, SQLite 3.3 can read database files created with SQLite 3.2,
    but databases created with SQLite 3.3 cannot be read by SQLite 3.2.
    Please refer to the SQLite documentation and change logs for information about
    file format compatibility between versions.

    Qt minor releases usually follow the SQLite minor releases, while Qt patch releases
    follow SQLite patch releases. Patch releases are therefore both backward and forward
    compatible.

    To force SQLite to use a specific file format, it is neccessary to build and
    ship your own database plugin with your own SQLite library as illustrated above.
    Some versions of SQLite can be forced to write a specific file format by setting
    the \c{SQLITE_DEFAULT_FILE_FORMAT} define when building SQLite.

    \target QIBASE
    \section2 QIBASE for Borland InterBase

    \section3 General Information about QIBASE

    The Qt InterBase plugin makes it possible to access the InterBase and
    Firebird databases. InterBase can either be used as a client/server or
    without a server in which case it operates on local files. The
    database file must exist before a connection can be established.

    Note that InterBase requires you to specify the full path to the
    database file, no matter whether it is stored locally or on another
    server.

    \snippet doc/src/snippets/code/doc_src_sql-driver.qdoc 24

    You need the InterBase/Firebird development headers and libraries
    to build this plugin.

    Due to license incompatibilities with the GPL, users of the Qt Open Source
    Edition are not allowed to link this plugin to the commercial editions of
    InterBase. Please use Firebird or the free edition of InterBase.

    \section3 QIBASE Unicode Support and Text Encoding

    By default the driver connects to the database using UNICODE_FSS. This can
    be overridden by setting the ISC_DPB_LC_CTYPE parameter with
    QSqlDatabase::setConnectOptions() before opening the connection.

    \snippet doc/src/snippets/code/doc_src_sql-driver.qdoc 25

    If Qt doesn't support the given text encoding the driver will issue a
    warning message and connect to the database using UNICODE_FSS.

    Note that if the text encoding set when connecting to the database is
    not the same as in the database, problems with transliteration might arise.

    \section3 QIBASE Stored procedures

    InterBase/Firebird return OUT values as result set, so when calling stored
    procedure, only IN values need to be bound via QSqlQuery::bindValue(). The
    RETURN/OUT values can be retrieved via QSqlQuery::value(). Example:

    \snippet doc/src/snippets/code/doc_src_sql-driver.qdoc 26

    \section3 How to Build the QIBASE Plugin on Unix and Mac OS X

    The following assumes InterBase or Firebird is installed in
    \c{/opt/interbase}:

    If you are using InterBase:

    \snippet doc/src/snippets/code/doc_src_sql-driver.qdoc 27

    If you are using Firebird, the Firebird library has to be set explicitly:

    \snippet doc/src/snippets/code/doc_src_sql-driver.qdoc 28

    \section3 How to Build the QIBASE Plugin on Windows

    The following assumes InterBase or Firebird is installed in
    \c{C:\interbase}:

    If you are using InterBase:

    \snippet doc/src/snippets/code/doc_src_sql-driver.qdoc 29

    If you are using Firebird, the Firebird library has to be set explicitely:

    \snippet doc/src/snippets/code/doc_src_sql-driver.qdoc 30

    If you are not using a Microsoft compiler, replace \c nmake
    with \c make in the line above.

    Note that \c{C:\interbase\bin} must be in the \c PATH.

    \bold{Note:} This database plugin is not supported for Windows CE.

    \target troubleshooting
    \section1 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 \c{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:

    \list
    \i Ensure that you are using a shared Qt library; you cannot use the
        plugins with a static build.
    \i Ensure that the plugin is in the correct directory. You can use
        QApplication::libraryPaths() to determine where Qt looks for plugins.
    \i Ensure that the client libraries of the DBMS are available on the
        system. On Unix, run the command \c{ldd} and pass the name of the
        plugin as parameter, for example \c{ldd libqsqlmysql.so}. You will
        get a warning if any of the client libraries couldn't be found.
        On Windows, you can use Visual Studio's dependency walker.
    \i Compile Qt with \c{QT_DEBUG_COMPONENT} defined to get very verbose
        debug output when loading plugins.
    \endlist

    Make sure you have followed the guide to \l{Deploying Plugins}.
    If you experience plugin load problems and see output like this:

    \snippet doc/src/snippets/code/doc_src_sql-driver.qdoc 31

    the problem is usually that the plugin had the wrong \l{Deploying
    Plugins#The Build Key}{build key}. This might require removing an
    entry from the \l{Deploying Plugins#The Plugin Cache} {plugin cache}.

    \target development
    \section1 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 \c{SELECT},
    \c{UPDATE}, and \c{ALTER} \c{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_PLUGIN2() macro. Read \l{How to Create Qt
    Plugins} for more information on this. You can also check out how
    this is done in the SQL plugins that are provided with Qt in
    \c{QTDIR/src/plugins/sqldrivers} and \c{QTDIR/src/sql/drivers}.

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

    \snippet doc/src/snippets/sqldatabase/sqldatabase.cpp 47
    \codeline
    \snippet doc/src/snippets/sqldatabase/sqldatabase.cpp 48
*/