tqt3/doc/sql-driver.doc

/****************************************************************************
**
** Documentation for sql driver programming
**
** Copyright (C) 1992-2008 Trolltech ASA.  All rights reserved.
**
** This file is part of the TQt GUI Toolkit.
**
** This file may be used under the terms of the GNU General
** Public License versions 2.0 or 3.0 as published by the Free
** Software Foundation and appearing in the files LICENSE.GPL2
** and LICENSE.GPL3 included in the packaging of this file.
** Alternatively you may (at your option) use any later version
** of the GNU General Public License if such license has been
** publicly approved by Trolltech ASA (or its successors, if any)
** and the KDE Free TQt Foundation.
**
** Please review the following information to ensure GNU General
** Public Licensing requirements will be met:
** http://trolltech.com/products/qt/licenses/licensing/opensource/.
** If you are unsure which license is appropriate for your use, please
** review the following information:
** http://trolltech.com/products/qt/licenses/licensing/licensingoverview
** or contact the sales department at sales@trolltech.com.
**
** This file may be used under the terms of the Q Public License as
** defined by Trolltech ASA and appearing in the file LICENSE.QPL
** included in the packaging of this file.  Licensees holding valid Qt
** Commercial licenses may use this file in accordance with the Qt
** Commercial License Agreement provided with the Software.
**
** This file is provided "AS IS" with NO WARRANTY OF ANY KIND,
** INCLUDING THE WARRANTIES OF DESIGN, MERCHANTABILITY AND FITNESS FOR
** A PARTICULAR PURPOSE. Trolltech reserves all rights not granted
** herein.
**
**********************************************************************/

/*! \page sql-driver.html

\title SQL Module - Drivers

\list
\i \link #Introduction Introduction\endlink
\i \link #building Building the drivers using configure\endlink
\i \link #buildingmanually Building the plugins manually\endlink
    \list
    \i \link #QDB2 QDB2\endlink - IBM DB2 Driver (v7.1 and higher)
    \i \link #QIBASE QIBASE\endlink - Borland Interbase Driver
    \i \link #QMYSQL3 QMYSQL3\endlink - MySQL Driver
    \i \link #QOCI8 QOCI8\endlink - Oracle Call Interface Driver, version 8, 9 and 10
    \i \link #QODBC3 QODBC3\endlink - Open Database Connectivity Driver
    \i \link #QPSQL7 QPSQL7\endlink - PostgreSQL v6.x and v7.x Driver
    \i \link #QSQLITE QSQLITE\endlink - SQLite Driver
    \i \link #QTDS7 QTDS7\endlink - Sybase Adaptive Server
    \endlist
\i \link #troubleshooting Troubleshooting\endlink
\i \link #development How to write your own database driver\endlink
\endlist

\target Introduction
\section1 Introduction

The \link sql.html SQL Module\endlink uses driver \link
plugins-howto.html plugins\endlink 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 TQt and other drivers
can be added. The driver source code is supplied and can be used as a
model for \link #development writing your own drivers\endlink.

\e{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.

The drivers shipped with TQt are:
\list
\i \link #QDB2 QDB2\endlink - IBM DB2 Driver (v7.1 and higher)
\i \link #QIBASE QIBASE\endlink - Borland Interbase Driver
\i \link #QMYSQL3 QMYSQL3\endlink - MySQL Driver
\i \link #QOCI8 QOCI8\endlink - Oracle Call Interface Driver, version 8, 9 and 10
\i \link #QODBC3 QODBC3\endlink - Open Database Connectivity Driver
\i \link #QPSQL7 QPSQL7\endlink - PostgreSQL v6.x and v7.x Driver
\i \link #QSQLITE QSQLITE\endlink - SQLite Driver
\i \link #QTDS7 QTDS7\endlink - Sybase Adaptive Server
\endlist

Note that not all of the plugins are shipped with the TQt Open Source Edition
due to license incompatibilities with the GPL.

\target building
\section1 Building the drivers using configure

The TQt 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:

\code
Possible values for <driver>: [ mysql oci odbc psql tds ]
Auto-Detected on this system: [ mysql psql ]
\endcode

Note that on Windows, the configure script doesn't do any
auto-detection.

The 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 "-I" and "-L" switches. 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 -I parameter doesn't accept spaces in
filenames, so use the 8.3 name instead, i.e. 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 TQt 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 QMYSQL3
\section2 QMYSQL3 - MySQL 3.x and MySQL 4.x

\keyword QMYSQL3

\section3 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 \l http://www.mysql.com

\section3 How to build the plugin on Unix/Linux

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

Tell \link qmake-manual.book qmake\endlink 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}:

\code
cd $QTDIR/plugins/src/sqldrivers/mysql
qmake -o Makefile "INCLUDEPATH+=/usr/local/include" "LIBS+=-L/usr/local/lib -lmysqlclient" mysql.pro
make
\endcode

\section3 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{C:\MYSQL}):

\code
cd %QTDIR%\plugins\src\sqldrivers\mysql
qmake -o Makefile "INCLUDEPATH+=C:\MYSQL\INCLUDE" "LIBS+=C:\MYSQL\LIB\OPT\LIBMYSQL.LIB" mysql.pro
nmake
\endcode

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

\target QOCI8
\section2 QOCI8 - Oracle Call Interface (OCI)

\keyword QOCI8

\section3 General information

The TQt OCI plugin supports Oracle 8, 9 and 10. After
connecting to the Oracle server, the plugin will auto-detect the
database version and enable features accordingly.

\section3 Unicode support

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

\section3 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).

\section3 Know problems 

When a query is in forward only mode a call to QSqlQuery::last() will
position the query on the last record and return TRUE, but subsequent 
calls to QSqlQuery::value() will only return NULLs.

\section3 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:

\list
\i \c libclntsh.so (all versions)
\i \c libwtc8.so (only Oracle 8) or \c libwtc9.so (only Oracle 9)
\endlist

Tell \c qmake where to find the Oracle header files and shared
libraries (it is assumed that the variable \c $ORACLE_HOME points to
the directory where Oracle is installed) and run make:

If you are using Oracle 8:
\code
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
\endcode

For Oracle version 9:
\code
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
\endcode

For Oracle version 10:
\code
cd $QTDIR/plugins/src/sqldrivers/oci
qmake -o Makefile "INCLUDEPATH+=$ORACLE_HOME/rdbms/public $ORACLE_HOME/rdbms/demo" "LIBS+=-L$ORACLE_HOME/lib -lclntsh" oci.pro
make
\endcode

Note that some versions of the OCI 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 TQt libray
itself, or configure TQt with the option '-DQT_NO_LIBRARY_UNLOAD'.
For Oracle 9, it is possible to link to the static OCI library by
using "LIBS+=$ORACLE_HOME/lib/libclntst9.a".

\section3 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{C:\oracle}):

\code
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
\endcode

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

\code
set PATH=%PATH%;c:\oracle\bin
\endcode

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

\target QODBC3
\section2 QODBC3 - Open Database Connectivity (ODBC)

\keyword QODBC3

\section3 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 TQt 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 \c{#define
ODBC_CHECK_DRIVER} line from the file \c{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.

\section3 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.

\section3 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 \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}:

\code
cd $QTDIR/plugins/src/sqldrivers/odbc
qmake "INCLUDEPATH+=/usr/local/unixODBC/include" "LIBS+=-L/usr/local/unixODBC/lib -lodbc"
make
\endcode

\section3 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:

\code
cd %QTDIR%\plugins\src\sqldrivers\odbc
qmake -o Makefile odbc.pro
nmake
\endcode

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

\target QPSQL7
\section2 QPSQL7 - PostgreSQL version 6 and 7

\keyword QPSQL7

\section3 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 library (libpq) because it is more stable and still backwards
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 \l http://www.postgresql.org.

\section3 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.

\section3 BLOB support

Binary Large Objects are supported through the \c BYTEA field type in
PostgreSQL versions >= 7.1. Fields of type \c OID can be read, but not
written. Use the PostgreSQL command \c lo_import to insert binary data
into \c OID fields.

\section3 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 \c qmake find the PostgreSQL header files and shared
libraries, run \c qmake the following way (assuming that the
PostgreSQL sources can be found in \c{/usr/src/psql}):

\code
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
\endcode

\section3 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{C:\psql}, build the plugin as follows:

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

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

\target QTDS7
\section2 QTDS7 - Sybase Adaptive Server

\keyword QTDS7

\section3 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
  (\l{http://www.freetds.org}). Note that FreeTDS is not yet stable,
  so some functionality may not work as expected.

- 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}.

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

\code
cd $QTDIR/plugins/src/sqldrivers/tds
qmake -o Makefile "INCLUDEPATH=$SYBASE/include" "LIBS=-L$SYBASE/lib -lsybdb"
make
\endcode

\section3 How to build the 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:

\code
cd %QTDIR%\plugins\src\sqldrivers\tds
qmake -o Makefile "LIBS+=NTWDBLIB.LIB" tds.pro
nmake
\endcode

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}.

\target QDB2
\section2 QDB2 - IBM DB2 Driver (v7.1 or higher)

\keyword QDB2

\section3 General information

The TQt DB2 plugin makes it possible to access IBM DB2 databases. It
has been tested with IBM DB2 v7.1 and 7.2. You have to 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 plugin on Unix/Linux

\code
cd $QTDIR/plugins/src/sqldrivers/db2
qmake -o Makefile "INCLUDEPATH+=$DB2DIR/include" "LIBS+=-L$DB2DIR/lib -ldb2"
make
\endcode

\section3 How to build the 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:

\code
cd %QTDIR%\plugins\src\sqldrivers\db2
qmake -o Makefile "INCLUDEPATH+=<DB2 home>/sqllib/include" "LIBS+=<DB2 home>/sqllib/lib/db2cli.lib"
nmake
\endcode

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

\target QSQLITE
\section2 QSQLITE - SQLite Driver

\keyword QSQLITE

The TQt SQLite plugin makes it possible to access SQLite databases.
SQLite is an in-process database, meaning that it is not necessary
to have a database server. SQLite operates on a single file, which has
to be set as 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.

SQLite has no support for types, every value is treated as character data.
BLOBs are therefore not supported.

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

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

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

\code
cd $QTDIR/plugins/src/sqldrivers/sqlite
qmake -o Makefile "INCLUDEPATH+=$SQLITE/include" "LIBS+=-L$SQLITE/lib -lsqlite"
make
\endcode

\target QIBASE
\section2 QIBASE - Borland Interbase Driver

\keyword QIBASE

\section3 General information

The TQt 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 operating 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. 

\code
    myDatabase->setHostName("MyServer");
    myDatabase->setDatabaseName("C:\\test.gdb");
\endcode

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

Due to the GPL, users of the TQt 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 How to build the plugin on Unix/Linux

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

\code
cd $QTDIR/plugins/src/sqldrivers/ibase
qmake -o Makefile "INCLUDEPATH+=/opt/interbase/include" "LIBS+=-L/opt/interbase/lib" ibase.pro
make
\endcode

\section3 How to build the plugin on Windows

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

\code
cd %QTDIR%\plugins\src\sqldrivers\ibase
qmake -o Makefile "INCLUDEPATH+=C:\interbase\include" ibase.pro
nmake
\endcode

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

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

\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 TQt library; you cannot use the
    plugins with a static build.
\i Ensure that the environment variable \c QTDIR points to the right
    directory. Go to the \c{$QTDIR/plugins/sqldrivers} directory and
    make sure that the plugin exists in that directory.
\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 the dependency walker of Visual Studio.
\endlist

If you are experiencing problems with loading plugins, and see output
like this

\code
QSqlDatabase warning: QMYSQL3 driver not loaded
QSqlDatabase: available drivers: QMYSQL3
\endcode

the problem is probably that the plugin had the wrong build key. For
debugging purposes, remove the corresponding entry in the
$HOME/.qt/qt_plugins_(qtversion).rc file.

The next time you try to load this plugin, it will give you a more detailed
error message.

\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 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 TQt SQL driver as a plugin (so that it is recognized and
loaded by the TQt library at runtime), the driver must use the
\c Q_EXPORT_PLUGIN macro. Read the \link plugins-howto.html Qt
Plugin\endlink documentation for more information on this. You can
also check out how this is done in the SQL plugins that is provided
with TQt in \c{QTDIR/plugins/src/sqldrivers} and
\c{QTDIR/src/sql/drivers}.

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

\code
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 ) ); }
};
\endcode

*/