QSqlDatabase Class Reference

The QSqlDatabase class represents a connection to a database. More...

 #include <QSqlDatabase>

Public Functions

QSqlDatabase ()
QSqlDatabase ( const QSqlDatabase & other )
~QSqlDatabase ()
void close ()
bool commit ()
QString connectOptions () const
QString connectionName () const
QString databaseName () const
QSqlDriver * driver () const
QString driverName () const
QSqlQuery exec ( const QString & query = QString() ) const
QString hostName () const
bool isOpen () const
bool isOpenError () const
bool isValid () const
QSqlError lastError () const
QSql::NumericalPrecisionPolicy numericalPrecisionPolicy () const
bool open ()
bool open ( const QString & user, const QString & password )
QString password () const
int port () const
QSqlIndex primaryIndex ( const QString & tablename ) const
QSqlRecord record ( const QString & tablename ) const
bool rollback ()
void setConnectOptions ( const QString & options = QString() )
void setDatabaseName ( const QString & name )
void setHostName ( const QString & host )
void setNumericalPrecisionPolicy ( QSql::NumericalPrecisionPolicy precisionPolicy )
void setPassword ( const QString & password )
void setPort ( int port )
void setUserName ( const QString & name )
QStringList tables ( QSql::TableType type = QSql::Tables ) const
bool transaction ()
QString userName () const
QSqlDatabase & operator= ( const QSqlDatabase & other )

Static Public Members

QSqlDatabase addDatabase ( const QString & type, const QString & connectionName = QLatin1String( defaultConnection ) )
QSqlDatabase addDatabase ( QSqlDriver * driver, const QString & connectionName = QLatin1String( defaultConnection ) )
QSqlDatabase cloneDatabase ( const QSqlDatabase & other, const QString & connectionName )
QStringList connectionNames ()
bool contains ( const QString & connectionName = QLatin1String( defaultConnection ) )
QSqlDatabase database ( const QString & connectionName = QLatin1String( defaultConnection ), bool open = true )
QStringList drivers ()
bool isDriverAvailable ( const QString & name )
void registerSqlDriver ( const QString & name, QSqlDriverCreatorBase * creator )
void removeDatabase ( const QString & connectionName )

Protected Functions

QSqlDatabase ( const QString & type )
QSqlDatabase ( QSqlDriver * driver )

Detailed Description

The QSqlDatabase class represents 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.

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 (i.e., what kind of database will you access?) 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, when you call any static member function that takes the connection name argument, if you don't pass the connection name argument, the default connection is assumed. 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.

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.

Once a connection is established, you can call tables() to get the list of tables in the database, call primaryIndex() to get a table's primary index, and call record() to get meta-information about a table's fields (e.g., field names).

Note: QSqlDatabase::exec() is deprecated. Use QSqlQuery::exec() instead.

If the driver supports transactions, use transaction() to start a transaction, and commit() or rollback() to complete it. Use hasFeature() to ask if the driver supports transactions. Note: When using transactions, you must start the transaction before you create your query.

If an error occurrs, lastError() will return information about it.

Get the names of the available SQL drivers with drivers(). Check for the presence of a particular driver with isDriverAvailable(). If you have created your own custom driver, you must register it with registerSqlDriver().

See also QSqlDriver, QSqlQuery, QtSql Module, and Threads and the SQL Module.

Member Function Documentation

QSqlDatabase::QSqlDatabase ()

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

QSqlDatabase::QSqlDatabase ( const QSqlDatabase & other )

Creates a copy of other.

QSqlDatabase::QSqlDatabase ( const QString & type ) [protected]

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:

Driver TypeDescription
QDB2IBM DB2
QIBASEBorland InterBase Driver
QMYSQLMySQL Driver
QOCIOracle Call Interface Driver
QODBCODBC Driver (includes Microsoft SQL Server)
QPSQLPostgreSQL Driver
QSQLITESQLite version 3 or above
QSQLITE2SQLite version 2
QTDSSybase Adaptive Server

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

See also SQL Database Drivers, registerSqlDriver(), and drivers().

QSqlDatabase::QSqlDatabase ( QSqlDriver * driver ) [protected]

This is an overloaded function.

Creates a database connection using the given driver.

QSqlDatabase::~QSqlDatabase ()

Destroys the object and frees any allocated resources.

See also close().

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:

DriverClass nameConstructor argumentsFile to include
QPSQLQPSQLDriverPGconn *connectionqsql_psql.cpp
QMYSQLQMYSQLDriverMYSQL *connectionqsql_mysql.cpp
QOCIQOCIDriverOCIEnv *environment, OCISvcCtx *serviceContextqsql_oci.cpp
QODBCQODBCDriverSQLHANDLE environment, SQLHANDLE connectionqsql_odbc.cpp
QDB2QDB2SQLHANDLE environment, SQLHANDLE connectionqsql_db2.cpp
QTDSQTDSDriverLOGINREC *loginRecord, DBPROCESS *dbProcess, const QString &hostNameqsql_tds.cpp
QSQLITEQSQLiteDriversqlite *connectionqsql_sqlite.cpp
QIBASEQIBaseDriverisc_db_handle connectionqsql_ibase.cpp

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 QSqlDatabase::cloneDatabase ( const QSqlDatabase & other, const QString & connectionName ) [static]

Clones the database connection other and 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().

void QSqlDatabase::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().

bool QSqlDatabase::commit ()

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 active query using the database for a SELECT. Make the query inactive before doing the commit.

Call lastError() to get information about errors.

See also QSqlQuery::isActive(), QSqlDriver::hasFeature(), and rollback().

QString QSqlDatabase::connectOptions () const

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

See also setConnectOptions().

QString QSqlDatabase::connectionName () const

Returns the connection name, which may be empty. Note: The connection name is not the database name.

This function was introduced in Qt 4.4.

See also addDatabase().

QStringList QSqlDatabase::connectionNames () [static]

Returns a list containing the names of all connections.

Note: This function is thread-safe.

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

bool QSqlDatabase::contains ( const QString & connectionName = QLatin1String( defaultConnection ) ) [static]

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

Note: This function is thread-safe.

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

QSqlDatabase QSqlDatabase::database ( const QString & connectionName = QLatin1String( defaultConnection ), bool open = true ) [static]

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.

Note: This function is thread-safe.

See also isOpen() and Threads and the SQL Module.

QString QSqlDatabase::databaseName () const

Returns the connection's database name, which may be empty. Note: The database name is not the connection name.

See also setDatabaseName().

QSqlDriver * QSqlDatabase::driver () const

Returns the database driver used to access the database connection.

See also addDatabase() and drivers().

QString QSqlDatabase::driverName () const

Returns the connection's driver name.

See also addDatabase() and driver().

QStringList QSqlDatabase::drivers () [static]

Returns a list of all the available database drivers.

See also registerSqlDriver().

QSqlQuery QSqlDatabase::exec ( const QString & query = QString() ) const

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.

See also QSqlQuery and lastError().

QString QSqlDatabase::hostName () const

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

See also setHostName().

bool QSqlDatabase::isDriverAvailable ( const QString & name ) [static]

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

See also drivers().

bool QSqlDatabase::isOpen () const

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

bool QSqlDatabase::isOpenError () const

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

bool QSqlDatabase::isValid () const

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 true if "sales" connection exists

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

QSqlError QSqlDatabase::lastError () const

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().

See also QSqlError and QSqlQuery::lastError().

QSql::NumericalPrecisionPolicy QSqlDatabase::numericalPrecisionPolicy () const

Returns the current default precision policy for the database connection.

This function was introduced in Qt 4.6.

See also QSql::NumericalPrecisionPolicy, setNumericalPrecisionPolicy(), QSqlQuery::numericalPrecisionPolicy(), and QSqlQuery::setNumericalPrecisionPolicy().

bool QSqlDatabase::open ()

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

See also lastError(), setDatabaseName(), setUserName(), setPassword(), setHostName(), setPort(), and setConnectOptions().

bool QSqlDatabase::open ( const QString & user, const QString & password )

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.

See also lastError().

QString QSqlDatabase::password () const

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

See also setPassword().

int QSqlDatabase::port () const

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

See also setPort().

QSqlIndex QSqlDatabase::primaryIndex ( const QString & tablename ) const

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

See also tables() and record().

QSqlRecord QSqlDatabase::record ( const QString & tablename ) const

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.

void QSqlDatabase::registerSqlDriver ( const QString & name, QSqlDriverCreatorBase * creator ) [static]

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<MyDatabaseDriver>);
 QSqlDatabase db = QSqlDatabase::addDatabase("MYDRIVER");

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

See also drivers().

void QSqlDatabase::removeDatabase ( const QString & connectionName ) [static]

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.

Note: This function is thread-safe.

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

bool QSqlDatabase::rollback ()

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 active query using the database for a SELECT. Make the query inactive before doing the rollback.

Call lastError() to get information about errors.

See also QSqlQuery::isActive(), QSqlDriver::hasFeature(), and commit().

void QSqlDatabase::setConnectOptions ( const QString & options = QString() )

Sets database-specific options. This must be done before the connection is opened or it has no effect (or you can close() the connection, call this function 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:

ODBCMySQLPostgreSQL
  • SQL_ATTR_ACCESS_MODE
  • SQL_ATTR_LOGIN_TIMEOUT
  • SQL_ATTR_CONNECTION_TIMEOUT
  • SQL_ATTR_CURRENT_CATALOG
  • SQL_ATTR_METADATA_ID
  • SQL_ATTR_PACKET_SIZE
  • SQL_ATTR_TRACEFILE
  • SQL_ATTR_TRACE
  • SQL_ATTR_CONNECTION_POOLING
  • SQL_ATTR_ODBC_VERSION
  • CLIENT_COMPRESS
  • CLIENT_FOUND_ROWS
  • CLIENT_IGNORE_SPACE
  • CLIENT_SSL
  • CLIENT_ODBC
  • CLIENT_NO_SCHEMA
  • CLIENT_INTERACTIVE
  • UNIX_SOCKET
  • MYSQL_OPT_RECONNECT
  • connect_timeout
  • options
  • tty
  • requiressl
  • service
DB2OCITDS
  • SQL_ATTR_ACCESS_MODE
  • SQL_ATTR_LOGIN_TIMEOUT
  • OCI_ATTR_PREFETCH_ROWS
  • OCI_ATTR_PREFETCH_MEMORY
none
SQLiteInterbase
  • QSQLITE_BUSY_TIMEOUT
  • QSQLITE_OPEN_READONLY
  • QSQLITE_ENABLE_SHARED_CACHE
  • ISC_DPB_LC_CTYPE
  • ISC_DPB_SQL_ROLE_NAME

Examples:

 ...
 // 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
     ...
 }

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

See also connectOptions().

void QSqlDatabase::setDatabaseName ( const QString & name )

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

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:

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

There is no default value.

See also databaseName(), setUserName(), setPassword(), setHostName(), setPort(), setConnectOptions(), and open().

void QSqlDatabase::setHostName ( const QString & host )

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.

See also hostName(), setUserName(), setPassword(), setDatabaseName(), setPort(), setConnectOptions(), and open().

void QSqlDatabase::setNumericalPrecisionPolicy ( QSql::NumericalPrecisionPolicy precisionPolicy )

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.

This function was introduced in Qt 4.6.

See also QSql::NumericalPrecisionPolicy, numericalPrecisionPolicy(), QSqlQuery::setNumericalPrecisionPolicy(), and QSqlQuery::numericalPrecisionPolicy().

void QSqlDatabase::setPassword ( const QString & password )

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.

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.

See also password(), setUserName(), setDatabaseName(), setHostName(), setPort(), setConnectOptions(), and open().

void QSqlDatabase::setPort ( int port )

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.

See also port(), setUserName(), setPassword(), setHostName(), setDatabaseName(), setConnectOptions(), and open().

void QSqlDatabase::setUserName ( const QString & name )

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.

See also userName(), setDatabaseName(), setPassword(), setHostName(), setPort(), setConnectOptions(), and open().

QStringList QSqlDatabase::tables ( QSql::TableType type = QSql::Tables ) const

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

See also primaryIndex() and record().

bool QSqlDatabase::transaction ()

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

See also QSqlDriver::hasFeature(), commit(), and rollback().

QString QSqlDatabase::userName () const

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

See also setUserName().

QSqlDatabase & QSqlDatabase::operator= ( const QSqlDatabase & other )

Assigns other to this object.

Notes provided by the Qt Community
Informative
  • 5

Votes: 2

Coverage: Qt library 4.7, 4.8, 5.0

Picture of Lukas Geyer Lukas Geyer

Lab Rat
4 notes

QSqlDatabase::open() closes and re-opens existing database connections

Be aware that QSqlDatabase::open() will close and re-open an existing database connection. As closing [developer.qt.nokia.com] a database connection leads to … 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. … calling QSqlDatabase::open() on an already opened database connection will invalidate any active queries which is known to cause memory leaks.

Do not call QSqlDatabase::open() on already opened database connections or make sure all existing queries are gone out of scope or have been deactivated.

[Revisions]

Informative
  • 5

Votes: 1

Coverage: Qt library 4.7, Qt 4.8

Picture of Zark Zark

Lab Rat
4 notes

SQLite transactions are not safe in multi-threading environment out of the box

For the SQLite driver the function QSqlDatabase::transaction() starts the transaction with “BEGIN” which by default is the DEFERRED mode. For safe usage of transactions by multiple threads simultaneously one should manually start transaction with “BEGIN IMMEDIATE TRANSACTION” query. Furthermore one should use this mode whenever transactions are used across threads in the application. Read more on SQLite transaction types here: http://www.sqlite.org/lang_transaction.html [sqlite.org]

[Revisions]

Informative
  • 4

Votes: 4

Coverage: Qt library 4.7, 4.8, 5.0

Picture of Volker Volker

Ant Farmer
35 notes

Nokia Certified Qt Developer

SQLite/Borland Interbase: setDatabaseName() takes full path to database file

In case of SQLite (QSQLITE) and Borland Interbase (QIBASE) – and only there – setDatabaseName() takes the full path to the database file. It may be a single file name, in that case it is located in “the current directory”, or some relative or absolute path + file name.

To make it relative to your application binary, you can use something like this:

  1. QSqlDatabase db = QSqlDatabase::addDatabase("QSQLITE");
  2. QString dbPath = QCoreApplication::applicationDirPath() + "/MyDatabase.db";
  3. db.setDatabaseName(dbPath);
  4. if (!db.open()) {
  5.     // handle error here...
  6. }

setHostName(), setPort(), setUsername() and setPassword() do not have any effect on SQLite databases and should be avoided.

[Revisions]

Informative
  • 3

Votes: 1

Coverage: Qt library 4.7, 4.8, 5.0

Picture of Rahul Das Rahul Das

Hobby Entomologist
7 notes

Note For Symbian Developers

void QSqlDatabase::setDatabaseName ( const QString & name ) on symbian shows various result. The separators are creating the trouble, even though forward or backward slashes are supposed to behave alike in Qt.

Converting separators to native form as follows with the help of QFileInfo class.

  1. QFileInfo fileInfo(fileName);
  2. ::setDatabaseName(QDir::toNativeSeparators(fileInfo.absoluteFilePath()));

[Revisions]

Informative
  • 1

Votes: 1

Coverage: Qt library 4.7, 4.8

Picture of Thanatos.jsse Thanatos.jsse

Lab Rat
1 note

Note for create SQL conection

Never use QSqlDatabase::addDatabase() for QSqlDatabase object created with new estament, if you do, you will have the following error:

  1. myDb->addDatabase("QPSQL");
  2. myDb->setHostName("acidalia");
  3. myDb->setDatabaseName("customdb");
  4. myDb->setUserName("mojito");
  5. myDb->setPassword("J0a1m8");
  6. qDebug() << QSqlDatabase::database().lastError().text();
  7. // "Driver not loaded Driver not loaded"

[Revisions]

Informative
  • 0

Votes: 0

Coverage: Qt library 4.8

Picture of d2uriel d2uriel

Lab Rat
1 note

Managing QtSql connections to MS SQL Server 2000 Desktop Engine (MSDE) through ODBC

This is a short example of managing connections to MSDE through ODBC. I also present a method of retrieving available databases on the server. Example below lacks some basic checks which should be implemented in a real life application, but it’s only supposed to show the way of actually working with QtSql module. The reason for creating this simple tutorial is that it took me some time to find out how to do all these things properly. You can find a thread created by me here [qt-project.org]. Please also note that this is brain to terminal so you may encounter some typos. Most of the code was taken from my application though, so it should actually work ;-).

I’d also like to note that I’m a beginner with Qt but I really tried to make this note as good as possible.

To start, we’ll create a SqlConnectionManager class which will take care of all SQL connections in our application.

SqlConnectionManager.h

  1. //! SqlConnectionManager class will manage your SQL connections.
  2. /**
  3.  * All opened connections will be kept alive throuout your application run time.
  4.  * Running queries is shown in at the end of this doc note.
  5.  */
  6. class SqlConnectionManager {
  7.    
  8. public:
  9.     SqlConnectionManager() {};
  10.     ~SqlConnectionManager();
  11.  
  12.     bool connect(QString ipAddress, QString databaseName);
  13.     QSqlDatabase* database(QString databaseName);
  14.  
  15. private:
  16.     QMap<QString, QSqlDatabase*> _connections;
  17.    
  18. };

SqlConnectionManager.cpp

  1. //! Destructor will take care of closing all connections and cleaning up resources.
  2. /**
  3.  * If you will not do it this way then you may receive warning messages like:
  4.  *
  5.  * QSqlDatabasePrivate::removeDatabase: connection 'your_connection_name'
  6.  * is still in use, all queries will cease to work.
  7.  */
  8. SqlConnectionManager::~SqlConnectionManager() {
  9.  
  10.     // Delete all QSqlDatabase objects first.
  11.     QMap<QString, QSqlDatabase*>::iterator it = _connections.begin();
  12.     while(!_connections.empty()) {
  13.         QSqlDatabase* tmp = it.value();
  14.         it = _connections.erase(it);
  15.         delete tmp;
  16.     }
  17.    
  18.     // Remove all databases.
  19.     QStringList connectionList = QSqlDatabase::connectionNames();
  20.     for(int i = 0; i < connectionList.count(); ++i) {
  21.         QSqlDatabase::removeDatabase(connectionList[i]);
  22.     }
  23. }
  24.  
  25. //! Connects to a database specified by databaseName on server with IP ipAddress.
  26. bool SqlConnectionManager::connect(QString ipAddress, QString databaseName) {
  27.  
  28.     // Second parameter in the function below is the connection name we will be using.
  29.     // Personally, I use static QString's for database names which are also used for connection names.
  30.  
  31.     QSqlDatabase* db = new QSqlDatabase(QSqlDatabase::addDatabase("QODBC", databaseName));
  32.     db->setDatabaseName("DRIVER={SQL SERVER}; SERVER={" + ipAddress + "}; DATABASE={" + databaseName + "}");
  33.     if(db->open()) {
  34.         // Connection was estabilished successfully.
  35.  
  36.         _connections.insert(databaseName, db);
  37.         return true;
  38.     } else {
  39.         // Failed to connect.
  40.  
  41.         return false;
  42.     }
  43. }
  44.  
  45. //! Returns a pointer to QSqlDatabase object which is isung connection name databaseName.
  46. QSqlDatabase* SqlConnectionManager::database(QString databaseName) {
  47.     return _connections[databaseName];
  48. }

Having the code above we can connect to your MSDE instance. Let’s say you have a TestDB on your own compuer with IP address 127.0.0.1. Here’s how we connect to it and run a query.

  1. const QString DBNAME = "TestDB";
  2.  
  3. SqlConnectionManager *sql = new SqlConnectionManager();
  4. if(sql->connect("127.0.0.1", DBNAME)) {
  5.     // Let's check if the connection is valid and if it's opened.
  6.  
  7.     if(sql->database(DBNAME)->isValid() && sql->database(DBNAME)->isOpen()) {
  8.         QSqlQuery *query = new QSqlQuery(*sql->database(DBNAME));
  9.         if(query->exec("SELECT id, testString FROM TestTable")) {
  10.             // Query executed successfully.
  11.             while(query->next()) {
  12.                 // Do something with the results.
  13.             }
  14.         } else {
  15.             qDebug() << "There was a problem executing the query.";
  16.         }
  17.  
  18.         // Remember to delete the query! Otherwise you will have memory leaks.
  19.         delete query;
  20.     } else {
  21.         qDebug() <<  "Connection was not valid or was closed.";
  22.     }
  23. }
  24.  
  25. // After deleting the SqlConnectionManager object all opened connections are closed and databases removed.
  26. delete sql;

Now in my application I had to list all available databases on MSDE. This is how I’ve done it:

In the consrtuctor of my class there’s a declaration:

  1. QSqlDatabase* dbms = new QSqlDatabase(QSqlDatabase::addDatabase("QODBC"));

Please note that I haven’t specified a connection name! It will use the default name.

Then I have a method which connects to all DB’s I need. Part of it looks like below:

  1. // Note there's no DATABASE parameter below!
  2. dbms->setDatabaseName("DRIVER={SQL SERVER}; SERVER={127.0.0.1}");
  3. if(!dbms->open()) {
  4.     // If I fail to open the DB my method returns false telling the caller that there's nothing he can do.
  5.     qDebug() << "Error connecting to SQL Server.";
  6.     return false;
  7. }

After all of that we can finally get a list of all databases.

  1. //! Method returns a QList of QStrings with names of all available databases on the server.
  2. QList<QString> backupEngine::getAvailableDBs() {
  3.     QList<QString> databases;
  4.     if(dbms->isValid() && dbms->isOpen()) {
  5.         QSqlQuery* query = new QSqlQuery(*dbms);
  6.         if (query->exec("SELECT name FROM master.dbo.sysdatabases")) {
  7.             while (query->next()) {
  8.                 databases.append(query->value(0).toString());
  9.             }
  10.         }
  11.         else {
  12.             qDebug() << "Failed getting list of databases.";
  13.         }
  14.         delete query;
  15.     } else {
  16.         qDebug() << "Lost connection to local database.";
  17.     }
  18.     return databases;
  19. }

[Revisions]