NAME
QSqlQuery - Means of executing and manipulating SQL
statements
SYNOPSIS
#include <qsqlquery.h>
Inherited by QSqlCursor.
Public Members
QSqlQuery ( QSqlResult * r )
QSqlQuery ( const QString & query = QString::null,
QSqlDatabase * db = 0 )
explicit QSqlQuery ( QSqlDatabase * db )
QSqlQuery ( const QSqlQuery & other )
QSqlQuery & operator= ( const QSqlQuery & other )
virtual ~QSqlQuery ()
bool isValid () const
bool isActive () const
bool isNull ( int field ) const
int at () const
QString lastQuery () const
int numRowsAffected () const
QSqlError lastError () const
bool isSelect () const
int size () const
const QSqlDriver * driver () const
const QSqlResult * result () const
bool isForwardOnly () const
void setForwardOnly ( bool forward )
virtual bool exec ( const QString & query )
virtual QVariant value ( int i ) const
virtual bool seek ( int i, bool relative = FALSE )
virtual bool next ()
virtual bool prev ()
virtual bool first ()
virtual bool last ()
bool exec ()
bool prepare ( const QString & query )
void bindValue ( const QString & placeholder, const QVariant
& val )
void bindValue ( int pos, const QVariant & val )
void addBindValue ( const QVariant & val )
void bindValue ( const QString & placeholder, const QVariant
& val, QSql::ParameterType type )
void bindValue ( int pos, const QVariant & val,
QSql::ParameterType type )
void addBindValue ( const QVariant & val,
QSql::ParameterType type )
QVariant boundValue ( const QString & placeholder ) const
QVariant boundValue ( int pos ) const
QMap<QString, QVariant> boundValues () const
QString executedQuery () const
Protected Members
virtual void beforeSeek ()
virtual void afterSeek ()
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:
next()
prev()
first()
last()
seek(int)
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 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:
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:
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:
immediately after a query is executed, an attempt is made to
retrieve the first record.
If the result is currently located before the first record, e.g.
is no change and FALSE is returned.
If the result is currently located after the last record, there
made to retrieve the next record.
If the result is located somewhere in the middle, an attempt is
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:
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:
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:
is no change and FALSE is returned.
If the result is currently located before the first record, there
attempt is made to retrieve the last record.
If the result is currently located after the last record, an
retrieve the previous record.
If the result is somewhere in the middle, an attempt is made to
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:
record and FALSE is returned.
If i is negative, the result is positioned before the first
i. If the record at position i 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.
Otherwise, an attempt is made to move to the record at position
If relative is TRUE, the following rules apply:
on the first record, and i is negative, there is no change, and
FALSE is returned.
If the result is currently positioned before the first record or
is positive, there is no change, and FALSE is returned.
If the result is currently located after the last record, and i
the relative offset i moves the result below zero, the result is
positioned before the first record and FALSE is returned.
If the result is currently located somewhere in the middle, and
ahead of the current record (or i records behind the current
record if i is negative). If the record at offset i could not be
retrieved, the result is positioned after the last record if i >=
0, (or before the first record if i is negative), and FALSE is
returned. If the record is successfully retrieved, TRUE is
returned.
Otherwise, an attempt is made to move to the record i records
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:
SEE ALSO
http://doc.trolltech.com/qsqlquery.html
http://www.trolltech.com/faq/tech.html
COPYRIGHT
Copyright 1992-2001 Trolltech AS, http://www.trolltech.com.
See the license file included in the distribution for a
complete license statement.
AUTHOR
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