QSqlDatabase Class

See also QSqlDriver, QSqlQuery, Qt SQL, Threads and the SQL Module

Creates an empty, invalid QSqlDatabase object. Use addDatabase(), removeDatabase(), and database() to get valid QSqlDatabase objects.

This is an overloaded function.

Creates a QSqlDatabase connection that uses the driver referred to by type. If the type is not recognized, the database connection will have no functionality.

The currently available driver types are:

Additional third party drivers, including your own custom drivers, can be loaded dynamically.

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This is an overloaded function.

Creates a database connection using the given driver.

Creates a copy of other.

Destroys the object and frees any allocated resources.

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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.

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().

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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:

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.

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:

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Clones the database connection other and stores it as connectionName. All the settings from the original database, e.g. databaseName(), hostName(), etc., are copied across. Does nothing if other is an invalid database. Returns the newly created database connection.

This is an overloaded function.

Clones the database connection other and stores it as connectionName. All the settings from the original database, e.g. databaseName(), hostName(), etc., are copied across. Does nothing if other is an invalid database. Returns the newly created database connection.

This overload is useful when cloning the database in another thread to the one that is used by the database represented by other.

This function was introduced in Qt 5.13.

Closes the database connection, freeing any resources acquired, and invalidating any existing QSqlQuery objects that are used with the database.

This will also affect copies of this QSqlDatabase object.

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Commits a transaction to the database if the driver supports transactions and a transaction() has been started. Returns true if the operation succeeded. Otherwise it returns false.

Call lastError() to get information about errors.

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Returns the connection options string used for this connection. The string may be empty.

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Returns the connection name, which may be empty.

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Returns a list containing the names of all connections.

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Returns true if the list of database connections contains connectionName; otherwise returns false.

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Returns the database connection called connectionName. The database connection must have been previously added with addDatabase(). If open is true (the default) and the database connection is not already open it is opened now. If no connectionName is specified the default connection is used. If connectionName does not exist in the list of databases, an invalid connection is returned.

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Returns the connection's database name, which may be empty.

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Returns the database driver used to access the database connection.

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Returns the connection's driver name.

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Returns a list of all the available database drivers.

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Executes a SQL statement on the database and returns a QSqlQuery object. Use lastError() to retrieve error information. If query is empty, an empty, invalid query is returned and lastError() is not affected.

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Returns the connection's host name; it may be empty.

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Returns true if a driver called name is available; otherwise returns false.

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Returns true if the database connection is currently open; otherwise returns false.

Returns true if there was an error opening the database connection; otherwise returns false. Error information can be retrieved using the lastError() function.

Returns true if the QSqlDatabase has a valid driver.

Example:

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

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

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

Returns information about the last error that occurred on the database.

Failures that occur in conjunction with an individual query are reported by QSqlQuery::lastError().

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Returns the current default precision policy for the database connection.

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Opens the database connection using the current connection values. Returns true on success; otherwise returns false. Error information can be retrieved using lastError().

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This is an overloaded function.

Opens the database connection using the given user name and password. Returns true on success; otherwise returns false. Error information can be retrieved using the lastError() function.

This function does not store the password it is given. Instead, the password is passed directly to the driver for opening the connection and it is then discarded.

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Returns the connection's password. An empty string will be returned if the password was not set with setPassword(), and if the password was given in the open() call, or if no password was used.

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Returns the connection's port number. The value is undefined if the port number has not been set.

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Returns the primary index for table tablename. If no primary index exists, an empty QSqlIndex is returned.

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Returns a QSqlRecord populated with the names of all the fields in the table (or view) called tablename. The order in which the fields appear in the record is undefined. If no such table (or view) exists, an empty record is returned.

This function registers a new SQL driver called name, within the SQL framework. This is useful if you have a custom SQL driver and don't want to compile it as a plugin.

Example:

QSqlDatabase::registerSqlDriver("MYDRIVER", new QSqlDriverCreator<QSqlDriver>);
QVERIFY(QSqlDatabase::drivers().contains("MYDRIVER"));
QSqlDatabase db = QSqlDatabase::addDatabase("MYDRIVER");
QVERIFY(db.isValid());

QSqlDatabase takes ownership of the creator pointer, so you mustn't delete it yourself.

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Removes the database connection connectionName from the list of database connections.

Example:

// 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

The correct way to do it:

{
    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

To remove the default connection, which may have been created with a call to addDatabase() not specifying a connection name, you can retrieve the default connection name by calling connectionName() on the database returned by database(). Note that if a default database hasn't been created an invalid database will be returned.

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Rolls back a transaction on the database, if the driver supports transactions and a transaction() has been started. Returns true if the operation succeeded. Otherwise it returns false.

Call lastError() to get information about errors.

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Sets database-specific options. This must be done before the connection is opened, otherwise it has no effect. Another possibility is to close the connection, call QSqlDatabase::setConnectOptions(), and open() the connection again.

The format of the options string is a semicolon separated list of option names or option=value pairs. The options depend on the database client used:

Examples:

db.setConnectOptions("SSL_KEY=client-key.pem;SSL_CERT=client-cert.pem;SSL_CA=ca-cert.pem;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
    // ...
}
}

Refer to the client library documentation for more information about the different options.

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Sets the connection's database name to name. To have effect, the database name must be set before the connection is opened. Alternatively, you can close() the connection, set the database name, and call open() again.

For the QSQLITE driver, if the database name specified does not exist, then it will create the file for you unless the QSQLITE_OPEN_READONLY option is set.

Additionally, name can be set to ":memory:" which will create a temporary database which is only available for the lifetime of the application.

For the QOCI (Oracle) driver, the database name is the TNS Service Name.

For the QODBC driver, the name can either be a DSN, a DSN filename (in which case the file must have a .dsn extension), or a connection string.

For example, Microsoft Access users can use the following connection string to open an .mdb file directly, instead of having to create a DSN entry in the ODBC manager:

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

There is no default value.

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Sets the connection's host name to host. To have effect, the host name must be set before the connection is opened. Alternatively, you can close() the connection, set the host name, and call open() again.

There is no default value.

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Sets the default numerical precision policy used by queries created on this database connection to precisionPolicy.

Note: Drivers that don't support fetching numerical values with low precision will ignore the precision policy. You can use QSqlDriver::hasFeature() to find out whether a driver supports this feature.

Note: Setting the default precision policy to precisionPolicy doesn't affect any currently active queries.

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Sets the connection's password to password. To have effect, the password must be set before the connection is opened. Alternatively, you can close() the connection, set the password, and call open() again.

There is no default value.

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Sets the connection's port number to port. To have effect, the port number must be set before the connection is opened. Alternatively, you can close() the connection, set the port number, and call open() again..

There is no default value.

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Sets the connection's user name to name. To have effect, the user name must be set before the connection is opened. Alternatively, you can close() the connection, set the user name, and call open() again.

There is no default value.

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Returns a list of the database's tables, system tables and views, as specified by the parameter type.

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Begins a transaction on the database if the driver supports transactions. Returns true if the operation succeeded. Otherwise it returns false.

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Returns the connection's user name; it may be empty.

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Assigns other to this object.