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QSqlQuery Class Reference
[sql module]

The QSqlQuery class provides a means of executing and manipulating SQL statements. More...

#include <qsqlquery.h>

Inherited by QSqlCursor.

List of all member functions.

Public Members

Protected Members

Detailed Description

The QSqlQuery class provides a means of executing and manipulating SQL statements.

QSqlQuery encapsulates the functionality involved in creating, navigating and retrieving data from SQL queries which are executed on a QSqlDatabase. It can be used to execute DML (data manipulation language) statements, e.g. SELECT, INSERT, UPDATE and DELETE, and also DDL (data definition language) statements, e.g. CREATE TABLE. It can also be used to execute database-specific commands which are not standard SQL (e.g. SET DATESTYLE=ISO for PostgreSQL).

Successfully executed SQL statements set the query's state to active (isActive() returns TRUE); otherwise the query's state is set to inactive. In either case, when executing a new SQL statement, the query is positioned on an invalid record; an active query must be navigated to a valid record (so that isValid() returns TRUE) before values can be retrieved.

Navigating records is performed with the following functions:

These functions allow the programmer to move forward, backward or arbitrarily through the records returned by the query. If you only need to move forward through the results, e.g. using next() or using seek() with a positive offset, you can use setForwardOnly() and save a significant amount of memory overhead. Once an active query is positioned on a valid record, data can be retrieved using value(). All data is transferred from the SQL backend using QVariants.

For example: 

    QSqlQuery query( "SELECT name FROM customer" );
    while ( query.next() ) {
        QString name = query.value(0).toString();
        doSomething( name );
    }

To access the data returned by a query, use the value() method. Each field in the data returned by a SELECT statement is accessed by passing the field's position in the statement, starting from 0. For the sake of efficiency there are no methods to access a field by name. (The QSqlCursor class provides a higher level interface that generates SQL automatically and through which fields are accessible by name.)

QSqlQuery supports prepared query execution and binding of parameter values to placeholders. Since not all databases support these features, Qt emulates them when necessary. For example, the Oracle and ODBC drivers have proper prepared query support, and Qt makes use of it; but for databases that don't have this support, Qt implements the feature itself, e.g. by replacing placeholders with actual values when a query is executed.

Oracle databases identify placeholders by using a colon-name syntax, e.g :name. ODBC simply uses ? characters. Qt supports both syntaxes (although you can't mix them in the same query).

Below we present the same example using each of the four different binding approaches.

Named binding using named placeholders 

    QSqlQuery query;
    query.prepare( "INSERT INTO atable (id, forename, surname) VALUES (:id, :forename, :surname)" );
    query.bindValue( ":id", 1001 );
    query.bindValue( ":forename", "Bart" );
    query.bindValue( ":surname", "Simpson" );
    query.exec();

Positional binding using named placeholders 

    QSqlQuery query;
    query.prepare( "INSERT INTO atable (id, forename, surname) VALUES (:id, :forename, :surname)" );
    query.bindValue( 0, 1001 );
    query.bindValue( 1, "Bart" );
    query.bindValue( 2, "Simpson" );
    query.exec();

Binding values using positional placeholders #1 

    QSqlQuery query;
    query.prepare( "INSERT INTO atable (id, forename, surname) VALUES (?, ?, ?)" );
    query.bindValue( 0, 1001 );
    query.bindValue( 1, "Bart" );
    query.bindValue( 2, "Simpson" );
    query.exec();

Binding values using positional placeholders #2 

    query.prepare( "INSERT INTO atable (id, forename, surname) VALUES (?, ?, ?)" );
    query.addBindValue( 1001 );
    query.addBindValue( "Bart" );
    query.addBindValue( "Simpson" );
    query.exec();

See also QSqlDatabase, QSqlCursor, QVariant, and Database Classes.

Member Function Documentation

QSqlQuery::QSqlQuery ( QSqlResult * r )

Creates a QSqlQuery object which uses the QSqlResult r to communicate with a database.

QSqlQuery::QSqlQuery ( const QString & query = QString::null, QSqlDatabase * db = 0 )

Creates a QSqlQuery object using the SQL query and the database db. If db is 0, (the default), the application's default database is used. If query is not a null string, it will be executed.

See also QSqlDatabase.

explicit QSqlQuery::QSqlQuery ( QSqlDatabase * db )

Creates a QSqlQuery object using the database db. If db is 0, the application's default database is used.

See also QSqlDatabase.

QSqlQuery::QSqlQuery ( const QSqlQuery & other )

Constructs a copy of other.

QSqlQuery::~QSqlQuery () [virtual]

Destroys the object and frees any allocated resources.

void QSqlQuery::addBindValue ( const QVariant & val, QSql::ParameterType type )

Adds the value val to the list of values when using positional value binding. The order of the addBindValue() calls determines which placeholder a value will be bound to in the prepared query. If type is QSql::Out or QSql::InOut, the placeholder will be overwritten with data from the database after the exec() call.

See also bindValue(), prepare(), and exec().

void QSqlQuery::addBindValue ( const QVariant & val )

This is an overloaded member function, provided for convenience. It behaves essentially like the above function.

Binds the placeholder with type QSql::In.

void QSqlQuery::afterSeek () [virtual protected]

Protected virtual function called after the internal record pointer is moved to a new record. The default implementation does nothing.

int QSqlQuery::at () const

Returns the current internal position of the query. The first record is at position zero. If the position is invalid, a QSql::Location will be returned indicating the invalid position.

See also isValid().

Example: sql/overview/navigating/main.cpp.

void QSqlQuery::beforeSeek () [virtual protected]

Protected virtual function called before the internal record pointer is moved to a new record. The default implementation does nothing.

void QSqlQuery::bindValue ( const QString & placeholder, const QVariant & val, QSql::ParameterType type )

Set the placeholder placeholder to be bound to value val in the prepared statement. Note that the placeholder mark (e.g :) must be included when specifying the placeholder name. If type is QSql::Out or QSql::InOut, the placeholder will be overwritten with data from the database after the exec() call.

See also addBindValue(), prepare(), and exec().

void QSqlQuery::bindValue ( const QString & placeholder, const QVariant & val )

This is an overloaded member function, provided for convenience. It behaves essentially like the above function.

Binds the placeholder with type QSql::In.

void QSqlQuery::bindValue ( int pos, const QVariant & val )

This is an overloaded member function, provided for convenience. It behaves essentially like the above function.

Binds the placeholder at position pos with type QSql::In.

void QSqlQuery::bindValue ( int pos, const QVariant & val, QSql::ParameterType type )

This is an overloaded member function, provided for convenience. It behaves essentially like the above function.

Set the placeholder in position pos to be bound to value val in the prepared statement. Field numbering starts at 0. If type is QSql::Out or QSql::InOut, the placeholder will be overwritten with data from the database after the exec() call.

See also addBindValue(), prepare(), and exec().

QVariant QSqlQuery::boundValue ( const QString & placeholder ) const

Returns the value for the placeholder.

QVariant QSqlQuery::boundValue ( int pos ) const

This is an overloaded member function, provided for convenience. It behaves essentially like the above function.

Returns the value for the placeholder at position pos.

QMap<QString, QVariant> QSqlQuery::boundValues () const

Returns a map of the bound values.

The bound values can be examined the following way: 

    QSqlQuery query;
    ...
    // Examine the bound values - bound using named binding
    QMap<QString, QVariant>::ConstIterator it;
    QMap<QString, QVariant> vals = query.boundValues();
    for ( it = vals.begin(); it != vals.end(); ++it )
        qWarning( "Placeholder: " + it.key() + ", Value: " + (*it).toString() );
    ...
    
    // Examine the bound values - bound using positional binding
    QValueList<QVariant>::ConstIterator it;
    QValueList<QVariant> list = query.boundValues().values();
    int i = 0;
    for ( it = list.begin(); it != list.end(); ++it )
        qWarning( "Placeholder pos: %d, Value: " + (*it).toString(), i++ );
    ...

const QSqlDriver * QSqlQuery::driver () const

Returns the database driver associated with the query.

bool QSqlQuery::exec ( const QString & query ) [virtual]

Executes the SQL in query. Returns TRUE and sets the query state to active if the query was successful; otherwise returns FALSE and sets the query state to inactive. The query string must use syntax appropriate for the SQL database being queried, for example, standard SQL.

After the query is executed, the query is positioned on an invalid record, and must be navigated to a valid record before data values can be retrieved, e.g. using next().

Note that the last error for this query is reset when exec() is called.

See also isActive(), isValid(), next(), prev(), first(), last(), and seek().

Examples: sql/overview/basicbrowsing/main.cpp, sql/overview/basicbrowsing2/main.cpp, and sql/overview/basicdatamanip/main.cpp.

bool QSqlQuery::exec ()

This is an overloaded member function, provided for convenience. It behaves essentially like the above function.

Executes a previously prepared SQL query. Returns TRUE if the query executed successfully; otherwise returns FALSE.

See also prepare(), bindValue(), and addBindValue().

QString QSqlQuery::executedQuery () const

Returns the last query that was executed.

In most cases this function returns the same as lastQuery(). If a prepared query with placeholders is executed on a DBMS that does not support it, the preparation of this query is emulated. The placeholders in the original query are replaced with their bound values to form a new query. This function returns the modified query. Useful for debugging purposes.

See also lastQuery().

bool QSqlQuery::first () [virtual]

Retrieves the first record in the result, if available, and positions the query on the retrieved record. Note that the result must be in an active state and isSelect() must return TRUE before calling this function or it will do nothing and return FALSE. Returns TRUE if successful. If unsuccessful the query position is set to an invalid position and FALSE is returned.

Example: sql/overview/navigating/main.cpp.

bool QSqlQuery::isActive () const

Returns TRUE if the query is currently active; otherwise returns FALSE.

Examples: sql/overview/basicbrowsing/main.cpp, sql/overview/basicbrowsing2/main.cpp, sql/overview/basicdatamanip/main.cpp, sql/overview/navigating/main.cpp, and sql/overview/retrieve1/main.cpp.

bool QSqlQuery::isForwardOnly () const

Returns TRUE if you can only scroll forward through a result set; otherwise returns FALSE.

See also setForwardOnly().

bool QSqlQuery::isNull ( int field ) const

Returns TRUE if the query is active and positioned on a valid record and the field is NULL; otherwise returns FALSE. Note that for some drivers isNull() will not return accurate information until after an attempt is made to retrieve data.

See also isActive(), isValid(), and value().

bool QSqlQuery::isSelect () const

Returns TRUE if the current query is a SELECT statement; otherwise returns FALSE.

bool QSqlQuery::isValid () const

Returns TRUE if the query is currently positioned on a valid record; otherwise returns FALSE.

bool QSqlQuery::last () [virtual]

Retrieves the last record in the result, if available, and positions the query on the retrieved record. Note that the result must be in an active state and isSelect() must return TRUE before calling this function or it will do nothing and return FALSE. Returns TRUE if successful. If unsuccessful the query position is set to an invalid position and FALSE is returned.

Example: sql/overview/navigating/main.cpp.

QSqlError QSqlQuery::lastError () const

Returns error information about the last error (if any) that occurred.

See also QSqlError.

QString QSqlQuery::lastQuery () const

Returns the text of the current query being used, or QString::null if there is no current query text.

See also executedQuery().

bool QSqlQuery::next () [virtual]

Retrieves the next record in the result, if available, and positions the query on the retrieved record. Note that the result must be in an active state and isSelect() must return TRUE before calling this function or it will do nothing and return FALSE.

The following rules apply:

If the record could not be retrieved, the result is positioned after the last record and FALSE is returned. If the record is successfully retrieved, TRUE is returned.

See also at() and isValid().

Examples: sql/overview/basicbrowsing/main.cpp, sql/overview/basicbrowsing2/main.cpp, sql/overview/delete/main.cpp, sql/overview/order1/main.cpp, sql/overview/retrieve1/main.cpp, sql/overview/subclass4/main.cpp, and sql/overview/subclass5/main.cpp.

int QSqlQuery::numRowsAffected () const

Returns the number of rows affected by the result's SQL statement, or -1 if it cannot be determined. Note that for SELECT statements, the value is undefined. If the query is not active (isActive() returns FALSE), -1 is returned.

See also size() and QSqlDriver::hasFeature().

Examples: sql/overview/basicbrowsing2/main.cpp and sql/overview/basicdatamanip/main.cpp.

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

Assigns other to the query.

bool QSqlQuery::prepare ( const QString & query )

Prepares the SQL query query for execution. The query may contain placeholders for binding values. Both Oracle style colon-name (e.g. :surname), and ODBC style (e.g. ?) placeholders are supported; but they cannot be mixed in the same query. See the Description for examples.

See also exec(), bindValue(), and addBindValue().

bool QSqlQuery::prev () [virtual]

Retrieves the previous record in the result, if available, and positions the query on the retrieved record. Note that the result must be in an active state and isSelect() must return TRUE before calling this function or it will do nothing and return FALSE.

The following rules apply:

If the record could not be retrieved, the result is positioned before the first record and FALSE is returned. If the record is successfully retrieved, TRUE is returned.

See also at().

const QSqlResult * QSqlQuery::result () const

Returns the result associated with the query.

bool QSqlQuery::seek ( int i, bool relative = FALSE ) [virtual]

Retrieves the record at position (offset) i, if available, and positions the query on the retrieved record. The first record is at position 0. Note that the query must be in an active state and isSelect() must return TRUE before calling this function.

If relative is FALSE (the default), the following rules apply:

If relative is TRUE, the following rules apply:

Example: sql/overview/navigating/main.cpp.

void QSqlQuery::setForwardOnly ( bool forward )

Sets forward only mode to forward. If forward is TRUE only next() and seek() with positive values are allowed for navigating the results. Forward only mode needs far less memory since results do not have to be cached.

Forward only mode is off by default.

Forward only mode cannot be used with data aware widgets like QDataTable, since they must to be able to scroll backward as well as forward.

See also isForwardOnly(), next(), and seek().

int QSqlQuery::size () const

Returns the size of the result, (number of rows returned), or -1 if the size cannot be determined or the database does not support reporting information about query sizes. Note that for non-SELECT statements (isSelect() returns FALSE), size() will return -1. If the query is not active (isActive() returns FALSE), -1 is returned.

To determine the number of rows affected by a non-SELECT statement, use numRowsAffected().

See also isActive(), numRowsAffected(), and QSqlDriver::hasFeature().

Example: sql/overview/navigating/main.cpp.

QVariant QSqlQuery::value ( int i ) const [virtual]

Returns the value of the i-th field in the query (zero based).

The fields are numbered from left to right using the text of the SELECT statement, e.g. in SELECT forename, surname FROM people, field 0 is forename and field 1 is surname. Using SELECT * is not recommended because the order of the fields in the query is undefined.

An invalid QVariant is returned if field i does not exist, if the query is inactive, or if the query is positioned on an invalid record.

See also prev(), next(), first(), last(), seek(), isActive(), and isValid().

Examples: sql/overview/basicbrowsing/main.cpp, sql/overview/basicbrowsing2/main.cpp, sql/overview/retrieve1/main.cpp, sql/overview/subclass3/main.cpp, sql/overview/subclass4/main.cpp, sql/overview/subclass5/main.cpp, and sql/overview/table4/main.cpp.

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