QSqlDatabase Class Reference
[QtSql module]

QSqlDatabase QSqlDatabase::addDatabase ( const QString & type, const QString & connectionName = QLatin1String( defaultConnection ) )   [static]

Adds a database to the list of database connections using the driver type and the connection name connectionName. If there already exists a database connection called connectionName, that connection is removed.

The database connection is referred to by connectionName. The newly added database connection is returned.

If type is not available or could not be loaded, isValid() returns false.

If connectionName is not specified, the new connection becomes the default connection for the application, and subsequent calls to database() without the connection name argument will return the default connection. If a connectionName is provided here, use database(connectionName) to retrieve the connection.

Warning: If you add a connection with the same name as an existing connection, the new connection replaces the old one. If you call this function more than once without specifying connectionName, the default connection will be the one replaced.

Before using the connection, it must be initialized. e.g., call some or all of setDatabaseName(), setUserName(), setPassword(), setHostName(), setPort(), and setConnectOptions(), and, finally, open().

Note: This function is thread-safe.

See also database(), removeDatabase(), and Threads and the SQL Module.

QSqlDatabase QSqlDatabase::addDatabase ( QSqlDriver * driver, const QString & connectionName = QLatin1String( defaultConnection ) )   [static]

This overload is useful when you want to create a database connection with a driver you instantiated yourself. It might be your own database driver, or you might just need to instantiate one of the Qt drivers yourself. If you do this, it is recommended that you include the driver code in your application. For example, you can create a PostgreSQL connection with your own QPSQL driver like this:

 #include "qtdir/src/sql/drivers/psql/qsql_psql.cpp"

 PGconn *con = PQconnectdb("host=server user=bart password=simpson dbname=springfield");
 QPSQLDriver *drv =  new QPSQLDriver(con);
 QSqlDatabase db = QSqlDatabase::addDatabase(drv); // becomes the new default connection
 QSqlQuery query;
 query.exec("SELECT NAME, ID FROM STAFF");
 ...

The above code sets up a PostgreSQL connection and instantiates a QPSQLDriver object. Next, addDatabase() is called to add the connection to the known connections so that it can be used by the Qt SQL classes. When a driver is instantiated with a connection handle (or set of handles), Qt assumes that you have already opened the database connection.

Note: We assume that qtdir is the directory where Qt is installed. This will pull in the code that is needed to use the PostgreSQL client library and to instantiate a QPSQLDriver object, assuming that you have the PostgreSQL headers somewhere in your include search path.

Remember that you must link your application against the database client library. Make sure the client library is in your linker's search path, and add lines like these to your .pro file:

 unix:LIBS += -lpq
 win32:LIBS += libpqdll.lib

The method described works for all the supplied drivers. The only difference will be in the driver constructor arguments. Here is a table of the drivers included with Qt, their source code files, and their constructor arguments:

The host name (or service name) is needed when constructing the QTDSDriver for creating new connections for internal queries. This is to prevent blocking when several QSqlQuery objects are used simultaneously.

Warning: Adding a database connection with the same connection name as an existing connection, causes the existing connection to be replaced by the new one.

Warning: The SQL framework takes ownership of the driver. It must not be deleted. To remove the connection, use removeDatabase().

See also drivers().

 QSqlDatabase db;
 qDebug() << db.isValid();    // Returns false

 db = QSqlDatabase::database("sales");
 qDebug() << db.isValid();    // Returns true if "sales" connection exists

 QSqlDatabase::removeDatabase("sales");
 qDebug() << db.isValid();    // Returns false

 QSqlDatabase::registerSqlDriver("MYDRIVER",
                                 new QSqlDriverCreator<MyDatabaseDriver>);
 QSqlDatabase db = QSqlDatabase::addDatabase("MYDRIVER");

 // WRONG
 QSqlDatabase db = QSqlDatabase::database("sales");
 QSqlQuery query("SELECT NAME, DOB FROM EMPLOYEES", db);
 QSqlDatabase::removeDatabase("sales"); // will output a warning

 // "db" is now a dangling invalid database connection,
 // "query" contains an invalid result set

 {
     QSqlDatabase db = QSqlDatabase::database("sales");
     QSqlQuery query("SELECT NAME, DOB FROM EMPLOYEES", db);
 }
 // Both "db" and "query" are destroyed because they are out of scope
 QSqlDatabase::removeDatabase("sales"); // correct

 ...
 // MySQL connection
 db.setConnectOptions("CLIENT_SSL=1;CLIENT_IGNORE_SPACE=1"); // use an SSL connection to the server
 if (!db.open()) {
     db.setConnectOptions(); // clears the connect option string
     ...
 }
 ...
 // PostgreSQL connection
 db.setConnectOptions("requiressl=1"); // enable PostgreSQL SSL connections
 if (!db.open()) {
     db.setConnectOptions(); // clear options
     ...
 }
 ...
 // ODBC connection
 db.setConnectOptions("SQL_ATTR_ACCESS_MODE=SQL_MODE_READ_ONLY;SQL_ATTR_TRACE=SQL_OPT_TRACE_ON"); // set ODBC options
 if (!db.open()) {
     db.setConnectOptions(); // don't try to set this option
     ...
 }

 ...
 db = QSqlDatabase::addDatabase("QODBC");
 db.setDatabaseName("DRIVER={Microsoft Access Driver (*.mdb)};FIL={MS Access};DBQ=myaccessfile.mdb");
 if (db.open()) {
     // success!
 }
 ...