SQL Module - Drivers

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 ) ); }
};