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. 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 for that generates SQL automatically and through which
fields are accessed by name.)
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.
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::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.
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::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::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.
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 at() and isValid().
Examples: sql/overview/basicbrowsing/main.cpp, sql/overview/basicbrowsing2/main.cpp, sql/overview/del/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,
this value will be the same as size(). 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::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 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 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.
Example: sql/overview/navigating/main.cpp.
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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