QSqlQuery(3qt)QSqlQuery(3qt)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. Information
about the fields can be obtained via QSqlDatabase::record(). For the
sake of efficiency there are no functions to access a field by name.
(The QSqlCursor class provides a higher-level interface with field
access by name and automatic SQL generation.)
QSqlQuery supports prepared query execution and the binding of
parameter values to placeholders. Some databases don't support these
features, so for them Qt emulates the required functionality. 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. The exception
is positional binding using named placeholders, which requires that the
database supports prepared queries.
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();
Note: Using positional binding with named placeholders will only work
if the database supports prepared queries. This can be checked with
QSqlDriver::hasFeature() using QSqlDriver::PreparedQueries as argument
for driver feature.
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();
Binding values to a stored procedure This code calls a stored procedure
called AsciiToInt(), passing it a character through its in parameter,
and taking its result in the out parameter.
QSqlQuery query;
query.prepare( "call AsciiToInt(?, ?)" );
query.bindValue( 0, "A" );
query.bindValue( 1, 0, QSql::Out );
query.exec();
int i = query.boundValue( 1 ).toInt(); // i is 65.
See also QSqlDatabase, QSqlCursor, QVariant, and Database Classes.
MEMBER FUNCTION DOCUMENTATIONQSqlQuery::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 prev(), next(), first(), last(), seek(), isActive(), and
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 in 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.
See also next(), prev(), last(), seek(), at(), isActive(), and
isValid().
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.
See also next(), prev(), first(), seek(), at(), isActive(), and
isValid().
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 result is currently located before the first record, e.g.
immediately after a query is executed, an attempt is made to retrieve
the first record.
If the result is currently located after the last record, there is no
change and FALSE is returned.
If the result is located somewhere in the middle, an attempt is made to
retrieve the next record.
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 prev(), first(), last(), seek(), at(), isActive(), 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; see size() instead. 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:
If the result is currently located before the first record, there is no
change and FALSE is returned.
If the result is currently located after the last record, an attempt is
made to retrieve the last record.
If the result is somewhere in the middle, an attempt is made to
retrieve the previous record.
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 next(), first(), last(), seek(), at(), isActive(), and
isValid().
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 i is negative, the result is positioned before the first record and
FALSE is returned.
Otherwise, an attempt is made to move to the record at position 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.
If relative is TRUE, the following rules apply:
If the result is currently positioned before the first record or on the
first record, and i is negative, there is no change, and FALSE is
returned.
If the result is currently located after the last record, and i is
positive, there is no change, and FALSE is returned.
If the result is currently located somewhere in the middle, and the
relative offset i moves the result below zero, the result is positioned
before the first record and FALSE is returned.
Otherwise, an attempt is made to move to the record i records 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.
See also next(), prev(), first(), last(), at(), isActive(), and
isValid().
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 need 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 if 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
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Copyright 1992-2007 Trolltech ASA, http://www.trolltech.com. See the
license file included in the distribution for a complete license
statement.
AUTHOR
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Trolltech AS 2 February 2007 QSqlQuery(3qt)