QSqlDatabase — PyQt Documentation v6.9.0 (original) (raw)

PyQt6.QtSql.QSqlDatabase

Description¶

The QSqlDatabase class handles a connection to a database.

The QSqlDatabase class provides an interface for accessing a database through a connection. An instance of QSqlDatabase represents the connection. The connection provides access to the database via one of the supported database drivers, which are derived from QSqlDriver. Alternatively, you can subclass your own database driver from QSqlDriver. See How to Write Your Own Database Driver for more information. A QSqlDatabase instance must only be accessed by the thread it was created in. Therefore you have to make sure to create them in the correct context. Alternatively you can change the context with moveToThread().

Create a connection (i.e., an instance of QSqlDatabase) by calling one of the static addDatabase() functions, where you specify the driver or type of driver to use (depending on the type of database) and a connection name. A connection is known by its own name, not by the name of the database it connects to. You can have multiple connections to one database. QSqlDatabase also supports the concept of a default connection, which is the unnamed connection. To create the default connection, don’t pass the connection name argument when you call addDatabase(). Subsequently, the default connection will be assumed if you call any static member function without specifying the connection name. The following snippet shows how to create and open a default connection to a PostgreSQL database:

QSqlDatabase db = QSqlDatabase::addDatabase("QPSQL");

db.setHostName("acidalia");

db.setDatabaseName("customdb");

db.setUserName("mojito");

db.setPassword("J0a1m8");

bool ok = db.open();

Once the QSqlDatabase object has been created, set the connection parameters with setDatabaseName(), setUserName(), setPassword(), setHostName(), setPort(), and setConnectOptions(). Then call open() to activate the physical connection to the database. The connection is not usable until you open it.

The connection defined above will be the default connection, because we didn’t give a connection name to addDatabase(). Subsequently, you can get the default connection by calling database() without the connection name argument:

QSqlDatabase db = QSqlDatabase::database();

QSqlDatabase is a value class. Changes made to a database connection via one instance of QSqlDatabase will affect other instances of QSqlDatabase that represent the same connection. Use cloneDatabase() to create an independent database connection based on an existing one.

Warning: It is highly recommended that you do not keep a copy of the QSqlDatabase around as a member of a class, as this will prevent the instance from being correctly cleaned up on shutdown. If you need to access an existing QSqlDatabase, it should be accessed with database(). If you chose to have a QSqlDatabase member variable, this needs to be deleted before the QCoreApplication instance is deleted, otherwise it may lead to undefined behavior.

If you create multiple database connections, specify a unique connection name for each one, when you call addDatabase(). Use database() with a connection name to get that connection. Use removeDatabase() with a connection name to remove a connection. QSqlDatabase outputs a warning if you try to remove a connection referenced by other QSqlDatabase objects. Use contains() to see if a given connection name is in the list of connections.

Note: When using transactions, you must start the transaction before you create your query.

Methods¶

__init__()

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

__init__(QSqlDatabase)

Creates a copy of other.

__init__(Optional[str])

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.

__init__(QSqlDriver)

This is an overloaded function.

Creates a database connection using the given driver.

@staticmethod
addDatabase(Optional[str], connectionName: Optional[str] = '') → QSqlDatabase

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().

@staticmethod
addDatabase(QSqlDriver, connectionName: Optional[str] = '') → QSqlDatabase

This is an overloaded function.

This overload is useful when you want to create a database connection with a QSqlDriver 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.

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:

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().

@staticmethod
cloneDatabase(QSqlDatabase, Optional[str]) → QSqlDatabase

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.

Note: The new connection has not been opened. Before using the new connection, you must call open().

@staticmethod
cloneDatabase(Optional[str], Optional[str]) → QSqlDatabase

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.

Note: The new connection has not been opened. Before using the new connection, you must call open().

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

close()

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.

See also

removeDatabase().

commit() → bool

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.

Note: For some databases, the commit will fail and return false if there is an isActive() using the database for a SELECT. Make the query isActive() before doing the commit.

Call lastError() to get information about errors.

connectionName() → str

Returns the connection name, which may be empty.

Note: The connection name is not the databaseName().

@staticmethod
connectionNames() → list[str]

Returns a list containing the names of all connections.

See also

contains(), database().

connectOptions() → str

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

See also

setConnectOptions().

@staticmethod
contains(connectionName: Optional[str] = '') → bool

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

@staticmethod
database(connectionName: Optional[str] = '', open: bool = True) → QSqlDatabase

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.

databaseName() → str

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

Note: The database name is not the connection name.

See also

setDatabaseName().

driver() → QSqlDriver

Returns the database driver used to access the database connection.

See also

addDatabase(), drivers().

driverName() → str

Returns the connection’s driver name.

See also

addDatabase(), driver().

@staticmethod
drivers() → list[str]

Returns a list of all the available database drivers.

See also

registerSqlDriver().

exec(query: Optional[str] = '') → QSqlQuery

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.

Use exec() instead.

hostName() → str

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

@staticmethod
isDriverAvailable(Optional[str]) → bool

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

isOpen() → bool

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

isOpenError() → bool

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

isValid() → bool

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

lastError() → QSqlError

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

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

moveToThread(QThread) → bool

Changes the thread affinity for QSqlDatabase and its associated driver. This function returns true when the function succeeds. Event processing will continue in the targetThread.

During this operation you have to make sure that there is no QSqlQuery bound to this instance otherwise the QSqlDatabase will not be moved to the given thread and the function returns false.

Since the associated driver is derived from QObject, all constraints for moving a QObject to another thread also apply to this function.

numericalPrecisionPolicy() → NumericalPrecisionPolicy

Returns the numericalPrecisionPolicy.

open() → bool

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

open(Optional[str], Optional[str]) → bool

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.

password() → str

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.

port() → int

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

primaryIndex(Optional[str]) → QSqlIndex

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

Note: Some drivers, such as the QPSQL driver, may may require you to pass tablename in lower case if the table was not quoted when created. See the Qt SQL driver documentation for more information.

See also

tables(), record().

record(Optional[str]) → QSqlRecord

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.

Note: Some drivers, such as the QPSQL driver, may may require you to pass tablename in lower case if the table was not quoted when created. See the Qt SQL driver documentation for more information.

@staticmethod
registerSqlDriver(Optional[str], QSqlDriverCreatorBase)

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);

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.

@staticmethod
removeDatabase(Optional[str])

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

Warning: There should be no open queries on the database connection when this function is called, otherwise a resource leak will occur.

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.

rollback() → bool

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.

Note: For some databases, the rollback will fail and return false if there is an isActive() using the database for a SELECT. Make the query isActive() before doing the rollback.

Call lastError() to get information about errors.

setConnectOptions(options: Optional[str] = '')

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 and are described for each plugin in the SQL Database Drivers page.

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.

See also

connectOptions().

setDatabaseName(Optional[str])

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

Note: The database name is not the connection name. The connection name must be passed to addDatabase() at connection object create time.

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.

setHostName(Optional[str])

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

There is no default value.

setNumericalPrecisionPolicy(NumericalPrecisionPolicy)

Sets numericalPrecisionPolicy() to precisionPolicy.

setPassword(Optional[str])

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

There is no default value.

Warning: This function stores the password in plain text within Qt. Use the open() call that takes a password as parameter to avoid this behavior.

setPort(int)

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

There is no default value.

setUserName(Optional[str])

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

There is no default value.

tables(type: TableType = Tables) → list[str]

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

See also

primaryIndex(), record().

thread() → QThread

Returns a pointer to the associated QThread instance.

transaction() → bool

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

userName() → str

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