- Qt 6.5
- Qt SQL
- C++ Classes
- QSqlDriver
- Qt 6.5.0 Reference Documentation

# QSqlDriver Class

The QSqlDriver class is an abstract base class for accessing specific SQL databases. More...

Header: | #include <QSqlDriver>
CMake: | find_package(Qt6 REQUIRED COMPONENTS Sql) target_link_libraries(mytarget PRIVATE Qt6::Sql)
qmake: | QT += sql
Inherits: | QObject

- List of all members, including inherited members
- QSqlDriver is part of Database Classes.

## Public Types

enum | DriverFeature { Transactions, QuerySize, BLOB, Unicode, PreparedQueries, …, CancelQuery }
enum | IdentifierType { FieldName, TableName }
enum | NotificationSource { UnknownSource, SelfSource, OtherSource }
enum | StatementType { WhereStatement, SelectStatement, UpdateStatement, InsertStatement, DeleteStatement }

## Public Functions

| QSqlDriver(QObject *parent = nullptr)
virtual | ~QSqlDriver()
virtual bool | beginTransaction()
virtual void | close() = 0
virtual bool | commitTransaction()
virtual QSqlResult * | createResult() const = 0
virtual QString | escapeIdentifier(const QString &identifier, QSqlDriver::IdentifierType type) const
virtual QString | formatValue(const QSqlField &field, bool trimStrings = false) const
virtual QVariant | handle() const
virtual bool | hasFeature(QSqlDriver::DriverFeature feature) const = 0
virtual bool | isIdentifierEscaped(const QString &identifier, QSqlDriver::IdentifierType type) const
virtual bool | isOpen() const
bool | isOpenError() const
QSqlError | lastError() const
virtual int | maximumIdentifierLength(QSqlDriver::IdentifierType type) const
QSql::NumericalPrecisionPolicy | numericalPrecisionPolicy() const
virtual bool | open(const QString &db, const QString &user = QString(), const QString &password = QString(), const QString &host = QString(), int port = -1, const QString &options = QString()) = 0
virtual QSqlIndex | primaryIndex(const QString &tableName) const
virtual QSqlRecord | record(const QString &tableName) const
virtual bool | rollbackTransaction()
void | setNumericalPrecisionPolicy(QSql::NumericalPrecisionPolicy precisionPolicy)
virtual QString | sqlStatement(QSqlDriver::StatementType type, const QString &tableName, const QSqlRecord &rec, bool preparedStatement) const
virtual QString | stripDelimiters(const QString &identifier, QSqlDriver::IdentifierType type) const
virtual bool | subscribeToNotification(const QString &name)
virtual QStringList | subscribedToNotifications() const
virtual QStringList | tables(QSql::TableType tableType) const
virtual bool | unsubscribeFromNotification(const QString &name)

## Signals

void | notification(const QString &name, QSqlDriver::NotificationSource source, const QVariant &payload)

## Protected Functions

virtual void | setLastError(const QSqlError &error)
virtual void | setOpen(bool open)
virtual void | setOpenError(bool error)

## Detailed Description

This class should not be used directly. Use QSqlDatabase instead.

If you want to create your own SQL drivers, you can subclass this class and reimplement its pure virtual functions and those virtual functions that you need. See How to Write Your Own Database Driver for more information.

See also QSqlDatabase and QSqlResult.

## Member Type Documentation

### enum QSqlDriver::DriverFeature

This enum contains a list of features a driver might support. Use hasFeature() to query whether a feature is supported or not.

Constant | Value | Description
--- | --- | ---
`QSqlDriver::Transactions` | `0` | Whether the driver supports SQL transactions.
`QSqlDriver::QuerySize` | `1` | Whether the database is capable of reporting the size of a query. Note that some databases do not support returning the size (i.e. number of rows returned) of a query, in which case QSqlQuery::size() will return -1.
`QSqlDriver::BLOB` | `2` | Whether the driver supports Binary Large Object fields.
`QSqlDriver::Unicode` | `3` | Whether the driver supports Unicode strings if the database server does.
`QSqlDriver::PreparedQueries` | `4` | Whether the driver supports prepared query execution.
`QSqlDriver::NamedPlaceholders` | `5` | Whether the driver supports the use of named placeholders.
`QSqlDriver::PositionalPlaceholders` | `6` | Whether the driver supports the use of positional placeholders.
`QSqlDriver::LastInsertId` | `7` | Whether the driver supports returning the Id of the last touched row.
`QSqlDriver::BatchOperations` | `8` | Whether the driver supports batched operations, see QSqlQuery::execBatch()
`QSqlDriver::SimpleLocking` | `9` | Whether the driver disallows a write lock on a table while other queries have a read lock on it.
`QSqlDriver::LowPrecisionNumbers` | `10` | Whether the driver allows fetching numerical values with low precision.
`QSqlDriver::EventNotifications` | `11` | Whether the driver supports database event notifications.
`QSqlDriver::FinishQuery` | `12` | Whether the driver can do any low-level resource cleanup when QSqlQuery::finish() is called.
`QSqlDriver::MultipleResultSets` | `13` | Whether the driver can access multiple result sets returned from batched statements or stored procedures.
`QSqlDriver::CancelQuery` | `14` | Whether the driver allows cancelling a running query.

More information about supported features can be found in the Qt SQL driver documentation.

See also hasFeature().

### enum QSqlDriver::IdentifierType

This enum contains a list of SQL identifier types.

Constant | Value | Description
--- | --- | ---
`QSqlDriver::FieldName` | `0` | A SQL field name
`QSqlDriver::TableName` | `1` | A SQL table name

### enum QSqlDriver::NotificationSource

This enum contains a list of SQL notification sources.

Constant | Value | Description
--- | --- | ---
`QSqlDriver::UnknownSource` | `0` | The notification source is unknown
`QSqlDriver::SelfSource` | `1` | The notification source is this connection
`QSqlDriver::OtherSource` | `2` | The notification source is another connection

### enum QSqlDriver::StatementType

This enum contains a list of SQL statement (or clause) types the driver can create.

Constant | Value | Description
--- | --- | ---
`QSqlDriver::WhereStatement` | `0` | An SQL `WHERE` statement (e.g., `WHERE f = 5`).
`QSqlDriver::SelectStatement` | `1` | An SQL `SELECT` statement (e.g., `SELECT f FROM t`).
`QSqlDriver::UpdateStatement` | `2` | An SQL `UPDATE` statement (e.g., `UPDATE TABLE t set f = 1`).
`QSqlDriver::InsertStatement` | `3` | An SQL `INSERT` statement (e.g., `INSERT INTO t (f) values (1)`).
`QSqlDriver::DeleteStatement` | `4` | An SQL `DELETE` statement (e.g., `DELETE FROM t`).

See also sqlStatement().

## Member Function Documentation

### `[explicit]` QSqlDriver::QSqlDriver(QObject *parent = nullptr)

Constructs a new driver with the given parent.

### `[virtual]` QSqlDriver::~QSqlDriver()

Destroys the object and frees any allocated resources.

### `[virtual]` bool QSqlDriver::beginTransaction()

This function is called to begin a transaction. If successful, return true, otherwise return false. The default implementation does nothing and returns `false`.

See also commitTransaction() and rollbackTransaction().

### `[pure virtual]` void QSqlDriver::close()

Derived classes must reimplement this pure virtual function in order to close the database connection. Return true on success, false on failure.

See also open() and setOpen().

### `[virtual]` bool QSqlDriver::commitTransaction()

This function is called to commit a transaction. If successful, return true, otherwise return false. The default implementation does nothing and returns `false`.

See also beginTransaction() and rollbackTransaction().

### `[pure virtual]` QSqlResult *QSqlDriver::createResult() const

Creates an empty SQL result on the database. Derived classes must reimplement this function and return a QSqlResult object appropriate for their database to the caller.

### `[virtual]` QString QSqlDriver::escapeIdentifier(const QString &identifier, QSqlDriver::IdentifierType type) const

Returns the identifier escaped according to the database rules. identifier can either be a table name or field name, dependent on type.

The default implementation does nothing.

See also isIdentifierEscaped().

### `[virtual]` QString QSqlDriver::formatValue(const QSqlField &field, bool trimStrings = false) const

Returns a string representation of the field value for the database. This is used, for example, when constructing INSERT and UPDATE statements.

The default implementation returns the value formatted as a string according to the following rules:

- If field is character data, the value is returned enclosed in single quotation marks, which is appropriate for many SQL databases. Any embedded single-quote characters are escaped (replaced with two single-quote characters). If trimStrings is true (the default is false), all trailing whitespace is trimmed from the field.
- If field is date/time data, the value is formatted in ISO format and enclosed in single quotation marks. If the date/time data is invalid, "NULL" is returned.
- If field is bytearray data, and the driver can edit binary fields, the value is formatted as a hexadecimal string.
- For any other field type, toString() is called on its value and the result of this is returned.

See also QVariant::toString().

### `[virtual]` QVariant QSqlDriver::handle() const

Returns the low-level database handle wrapped in a QVariant or an invalid variant if there is no handle.

Warning: Use this with uttermost care and only if you know what you're doing.

Warning: The handle returned here can become a stale pointer if the connection is modified (for example, if you close the connection).

Warning: The handle can be NULL if the connection is not open yet.

The handle returned here is database-dependent, you should query the type name of the variant before accessing it.

This example retrieves the handle for a connection to sqlite:

     QSqlDatabase db = QSqlDatabase::database();
     QVariant v = db.driver()->handle();
     if (v.isValid() && (qstrcmp(v.typeName(), "sqlite3*") == 0)) {
         // v.data() returns a pointer to the handle
         sqlite3 *handle = *static_cast<sqlite3 **>(v.data());
         if (handle) {
             // ...
         }
     }

This snippet returns the handle for PostgreSQL or MySQL:

     if (qstrcmp(v.typeName(), "PGconn*") == 0) {
         PGconn *handle = *static_cast<PGconn **>(v.data());
         if (handle) {
             // ...
         }
     }

     if (qstrcmp(v.typeName(), "MYSQL*") == 0) {
         MYSQL *handle = *static_cast<MYSQL **>(v.data());
         if (handle) {
             // ...
         }
     }

See also QSqlResult::handle().

### `[pure virtual]` bool QSqlDriver::hasFeature(QSqlDriver::DriverFeature feature) const

Returns `true` if the driver supports feature feature; otherwise returns `false`.

Note that some databases need to be open() before this can be determined.

See also DriverFeature.

### `[virtual]` bool QSqlDriver::isIdentifierEscaped(const QString &identifier, QSqlDriver::IdentifierType type) const

Returns whether identifier is escaped according to the database rules. identifier can either be a table name or field name, dependent on type.

Reimplement this function if you want to provide your own implementation in your QSqlDriver subclass,

See also stripDelimiters() and escapeIdentifier().

### `[virtual]` bool QSqlDriver::isOpen() const

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

### bool QSqlDriver::isOpenError() const

Returns `true` if the there was an error opening the database connection; otherwise returns `false`.

### QSqlError QSqlDriver::lastError() const

Returns a QSqlError object which contains information about the last error that occurred on the database.

See also setLastError().

### `[virtual, since 6.0]` int QSqlDriver::maximumIdentifierLength(QSqlDriver::IdentifierType type) const

Returns the maximum length for the identifier type according to the database settings. Returns INT_MAX by default if the is no maximum for the database.

This function was introduced in Qt 6.0.

### `[signal]` void QSqlDriver::notification(const QString &name, QSqlDriver::NotificationSource source, const QVariant &payload)

This signal is emitted when the database posts an event notification that the driver subscribes to. name identifies the event notification, source indicates the signal source, payload holds the extra data optionally delivered with the notification.

See also subscribeToNotification().

### QSql::NumericalPrecisionPolicy QSqlDriver::numericalPrecisionPolicy() const

Returns the current default precision policy for the database connection.

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

### `[pure virtual]` bool QSqlDriver::open(const QString &db, const QString &user = QString(), const QString &password = QString(), const QString &host = QString(), int port = -1, const QString &options = QString())

Derived classes must reimplement this pure virtual function to open a database connection on database db, using user name user, password password, host host, port port and connection options options.

The function must return true on success and false on failure.

See also setOpen().

### `[virtual]` QSqlIndex QSqlDriver::primaryIndex(const QString &tableName) const

Returns the primary index for table tableName. Returns an empty QSqlIndex if the table doesn't have a primary index. The default implementation returns an empty index.

### `[virtual]` QSqlRecord QSqlDriver::record(const QString &tableName) const

Returns a QSqlRecord populated with the names of the fields in table tableName. If no such table exists, an empty record is returned. The default implementation returns an empty record.

### `[virtual]` bool QSqlDriver::rollbackTransaction()

This function is called to rollback a transaction. If successful, return true, otherwise return false. The default implementation does nothing and returns `false`.

See also beginTransaction() and commitTransaction().

### `[virtual protected]` void QSqlDriver::setLastError(const QSqlError &error)

This function is used to set the value of the last error, error, that occurred on the database.

See also lastError().

### void QSqlDriver::setNumericalPrecisionPolicy(QSql::NumericalPrecisionPolicy precisionPolicy)

Sets the default numerical precision policy used by queries created by this driver to precisionPolicy.

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

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

### `[virtual protected]` void QSqlDriver::setOpen(bool open)

This function sets the open state of the database to open. Derived classes can use this function to report the status of open().

See also open() and setOpenError().

### `[virtual protected]` void QSqlDriver::setOpenError(bool error)

This function sets the open error state of the database to error. Derived classes can use this function to report the status of open(). Note that if error is true the open state of the database is set to closed (i.e., isOpen() returns `false`).

See also isOpenError(), open(), and setOpen().

### `[virtual]` QString QSqlDriver::sqlStatement(QSqlDriver::StatementType type, const QString &tableName, const QSqlRecord &rec, bool preparedStatement) const

Returns a SQL statement of type type for the table tableName with the values from rec. If preparedStatement is true, the string will contain placeholders instead of values.

The generated flag in each field of rec determines whether the field is included in the generated statement.

This method can be used to manipulate tables without having to worry about database-dependent SQL dialects. For non-prepared statements, the values will be properly escaped.

In the WHERE statement, each non-null field of rec specifies a filter condition of equality to the field value, or if prepared, a placeholder. However, prepared or not, a null field specifies the condition IS NULL and never introduces a placeholder. The application must not attempt to bind data for the null field during execution. The field must be set to some non-null value if a placeholder is desired. Furthermore, since non-null fields specify equality conditions and SQL NULL is not equal to anything, even itself, it is generally not useful to bind a null to a placeholder.

### `[virtual]` QString QSqlDriver::stripDelimiters(const QString &identifier, QSqlDriver::IdentifierType type) const

Returns the identifier with the leading and trailing delimiters removed, identifier can either be a table name or field name, dependent on type. If identifier does not have leading and trailing delimiter characters, identifier is returned without modification.

Reimplement this function if you want to provide your own implementation in your QSqlDriver subclass,

See also isIdentifierEscaped().

### `[virtual]` bool QSqlDriver::subscribeToNotification(const QString &name)

This function is called to subscribe to event notifications from the database. name identifies the event notification.

If successful, return true, otherwise return false.

The database must be open when this function is called. When the database is closed by calling close() all subscribed event notifications are automatically unsubscribed. Note that calling open() on an already open database may implicitly cause close() to be called, which will cause the driver to unsubscribe from all event notifications.

When an event notification identified by name is posted by the database the notification() signal is emitted.

Reimplement this function if you want to provide event notification support in your own QSqlDriver subclass,

See also unsubscribeFromNotification(), subscribedToNotifications(), and QSqlDriver::hasFeature().

### `[virtual]` QStringList QSqlDriver::subscribedToNotifications() const

Returns a list of the names of the event notifications that are currently subscribed to.

Reimplement this function if you want to provide event notification support in your own QSqlDriver subclass,

See also subscribeToNotification() and unsubscribeFromNotification().

### `[virtual]` QStringList QSqlDriver::tables(QSql::TableType tableType) const

Returns a list of the names of the tables in the database. The default implementation returns an empty list.

The tableType argument describes what types of tables should be returned. Due to binary compatibility, the string contains the value of the enum QSql::TableTypes as text. An empty string should be treated as QSql::Tables for backward compatibility.

### `[virtual]` bool QSqlDriver::unsubscribeFromNotification(const QString &name)

This function is called to unsubscribe from event notifications from the database. name identifies the event notification.

If successful, return true, otherwise return false.

The database must be open when this function is called. All subscribed event notifications are automatically unsubscribed from when the close() function is called.

After calling this function the notification() signal will no longer be emitted when an event notification identified by name is posted by the database.

Reimplement this function if you want to provide event notification support in your own QSqlDriver subclass,

See also subscribeToNotification() and subscribedToNotifications().