The QSqlQuery class provides a means of executing and
manipulating SQL statements.
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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.
This makes using SELECT * queries inadvisable because the
order of the fields returned is indeterminate. 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 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.
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();
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 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.
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: 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.
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: 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.
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: 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; see size() instead. 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 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: 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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