Detailed Description
The QSqlCursor class provides browsing and editing of SQL
tables and views.
A QSqlCursor is a database record (see QSqlRecord) that
corresponds to a table or view within an SQL database (see QSqlDatabase). There are two buffers in a cursor, one used for
browsing and one used for editing records. Each buffer contains a
list of fields which correspond to the fields in the table or
view.
When positioned on a valid record, the browse buffer contains the
values of the current record's fields from the database. The edit
buffer is separate, and is used for editing existing records and
inserting new records.
For browsing data, a cursor must first select() data from the
database. After a successful select() the cursor is active
(isActive() returns TRUE), but is initially not positioned on a
valid record (isValid() returns FALSE). To position the cursor on
a valid record, use one of the navigation functions, next(),
prev(), first(), last(), or seek(). Once positioned on a valid
record, data can be retrieved from the browse buffer using
value(). If a navigation function is not successful, it returns
FALSE, the cursor will no longer be positioned on a valid record
and the values returned by value() are undefined.
For example:
QSqlCursor cur( "staff" ); // Specify the table/view name
cur.select(); // We'll retrieve every record
while ( cur.next() ) {
qDebug( cur.value( "id" ).toString() + ": " +
cur.value( "surname" ).toString() + " " +
cur.value( "salary" ).toString() );
}
In the above example, a cursor is created specifying a table or
view name in the database. Then, select() is called, which can be
optionally parameterised to filter and order the records
retrieved. Each record in the cursor is retrieved using next().
When next() returns FALSE, there are no more records to process,
and the loop terminates.
For editing records (rows of data), a cursor contains a separate
edit buffer which is independent of the fields used when browsing.
The functions insert(), update() and del() operate on the edit
buffer. This allows the cursor to be repositioned to other
records while simultaneously maintaining a separate buffer for
edits. You can get a pointer to the edit buffer using
editBuffer(). The primeInsert(), primeUpdate() and primeDelete()
functions also return a pointer to the edit buffer and prepare it
for insert, update and delete respectively. Edit operations only
affect a single row at a time. Note that update() and del()
require that the table or view contain a primaryIndex() to ensure
that edit operations affect a unique record within the database.
For example:
QSqlCursor cur( "prices" );
cur.select( "id=202" );
if ( cur.next() ) {
QSqlRecord *buffer = cur.primeUpdate();
double price = buffer->value( "price" ).toDouble();
double newprice = price * 1.05;
buffer->setValue( "price", newprice );
cur.update();
}
To edit an existing database record, first move to the record you
wish to update. Call primeUpdate() to get the pointer to the
cursor's edit buffer. Then use this pointer to modify the values
in the edit buffer. Finally, call update() to save the changes to
the database. The values in the edit buffer will be used to
locate the appropriate record when updating the database (see
primaryIndex()).
Similarly, when deleting an existing database record, first move
to the record you wish to delete. Then, call primeDelete() to get
the pointer to the edit buffer. Finally, call del() to delete the
record from the database. Again, the values in the edit buffer
will be used to locate and delete the appropriate record.
To insert a new record, call primeInsert() to get the pointer to
the edit buffer. Use this pointer to populate the edit buffer
with new values and then insert() the record into the database.
After calling insert(), update() or del(), the cursor is no longer
positioned on a valid record and can no longer be navigated
(isValid() return FALSE). The reason for this is that any changes
made to the database will not be visible until select() is called
to refresh the cursor. You can change this behavior by passing
FALSE to insert(), update() or del() which will prevent the cursor
from becoming invalid. The edits will still not be visible when
navigating the cursor until select() is called.
QSqlCursor contains virtual methods which allow editing behavior
to be customized by subclasses. This allows custom cursors to be
created that encapsulate the editing behavior of a database table
for an entire application. For example, a cursor can be customized
to always auto-number primary index fields, or provide fields with
suitable default values, when inserting new records. QSqlCursor
generates SQL statements which are sent to the database engine;
you can control which fields are included in these statements
using setGenerated().
Note that QSqlCursor does not inherit from QObject. This means
that you are responsible for destroying instances of this class
yourself. However if you create a QSqlCursor and use it in a
QDataTable, QDataBrowser or a QDataView these classes will
usually take ownership of the cursor and destroy it when they
don't need it anymore. The documentation for QDataTable,
QDataBrowser and QDataView explicitly states which calls take
ownership of the cursor.
See also Database Classes.
Member Type Documentation
QSqlCursor::Mode
This enum type describes how QSqlCursor operates on records in the
database.
- QSqlCursor::ReadOnly - the cursor can only SELECT records from the
database.
- QSqlCursor::Insert - the cursor can INSERT records into the database.
- QSqlCursor::Update - the cursor can UPDATE records in the database.
- QSqlCursor::Delete - the cursor can DELETE records from the database.
- QSqlCursor::Writable - the cursor can INSERT, UPDATE and DELETE records
in the database.
Member Function Documentation
QSqlCursor::QSqlCursor ( const QString & name = QString::null, bool autopopulate = TRUE, QSqlDatabase * db = 0 )
Constructs a cursor on database db using table or view name.
If autopopulate is TRUE (the default), the name of the
cursor must correspond to an existing table or view name in the
database so that field information can be automatically created.
If the table or view does not exist, the cursor will not be
functional.
The cursor is created with an initial mode of QSqlCursor::Writable
(meaning that records can be inserted, updated or deleted using
the cursor). If the cursor does not have a unique primary index,
update and deletes cannot be performed.
Note that autopopulate refers to populating the cursor with
meta-data, e.g. the names of the table's fields, not with
retrieving data. The select() function is used to populate the
cursor with data.
See also setName() and setMode().
QSqlCursor::QSqlCursor ( const QSqlCursor & other )
Constructs a copy of other.
QSqlCursor::~QSqlCursor ()
Destroys the object and frees any allocated resources.
void QSqlCursor::append ( const QSqlFieldInfo & fieldInfo ) [virtual]
Append a copy of field fieldInfo to the end of the cursor. Note
that all references to the cursor edit buffer become invalidated.
QVariant QSqlCursor::calculateField ( const QString & name ) [virtual protected]
Protected virtual function which is called whenever a field needs
to be calculated. If calculated fields are being used, derived
classes must reimplement this function and return the appropriate
value for field name. The default implementation returns an
invalid QVariant.
See also setCalculated().
Examples: sql/overview/subclass3/main.cpp and sql/overview/subclass4/main.cpp.
bool QSqlCursor::canDelete () const
Returns TRUE if the cursor will perform deletes; otherwise returns
FALSE.
See also setMode().
bool QSqlCursor::canInsert () const
Returns TRUE if the cursor will perform inserts; otherwise returns
FALSE.
See also setMode().
bool QSqlCursor::canUpdate () const
Returns TRUE if the cursor will perform updates; otherwise returns
FALSE.
See also setMode().
void QSqlCursor::clear () [virtual]
Removes all fields from the cursor. Note that all references to
the cursor edit buffer become invalidated.
Reimplemented from QSqlRecord.
int QSqlCursor::del ( bool invalidate = TRUE ) [virtual]
Deletes a record from the database using the cursor's primary
index and the contents of the cursor edit buffer. Returns the
number of records which were deleted.
For error information, use lastError().
Only records which meet the filter criteria specified by the
cursor's primary index are deleted. If the cursor does not contain
a primary index, no delete is performed and 0 is returned. If invalidate is TRUE (the default), the current cursor can no longer
be navigated. A new select() call must be made before you can move
to a valid record. For example:
QSqlCursor cur( "prices" );
cur.select( "id=999" );
if ( cur.next() ) {
cur.primeDelete();
cur.del();
}
In the above example, a cursor is created on the 'prices' table
and positioned to the record to be deleted. First primeDelete() is
called to populate the edit buffer with the current cursor values,
e.g. with an id of 999, and then del() is called to actually
delete the record from the database. Remember: all edit operations
(insert(), update() and delete()) operate on the contents of the
cursor edit buffer and not on the contents of the cursor itself.
See also primeDelete(), setMode(), and lastError().
Example: sql/overview/delete/main.cpp.
int QSqlCursor::del ( const QString & filter, bool invalidate = TRUE ) [virtual protected]
This is an overloaded member function, provided for convenience. It behaves essentially like the above function.
Deletes the current cursor record from the database using the
filter filter. Only records which meet the filter criteria are
deleted. Returns the number of records which were deleted. If invalidate is TRUE (the default), the current cursor can no longer
be navigated. A new select() call must be made before you can move
to a valid record. For error information, use lastError().
The filter is an SQL WHERE clause, e.g. id=500.
See also setMode() and lastError().
QSqlRecord * QSqlCursor::editBuffer ( bool copy = FALSE ) [virtual]
Returns the current internal edit buffer. If copy is TRUE (the
default is FALSE), the current cursor field values are first
copied into the edit buffer. The edit buffer is valid as long as
the cursor remains valid. The cursor retains ownership of the
returned pointer, so it must not be deleted or modified.
See also primeInsert(), primeUpdate(), and primeDelete().
QString QSqlCursor::filter () const
Returns the current filter, or an empty string if there is no
current filter.
QSqlIndex QSqlCursor::index ( const QStringList & fieldNames ) const [virtual]
Returns an index composed of fieldNames, all in ASCending
order. Note that all field names must exist in the cursor,
otherwise an empty index is returned.
See also QSqlIndex.
Examples: sql/overview/extract/main.cpp, sql/overview/order1/main.cpp, sql/overview/order2/main.cpp, and sql/overview/table3/main.cpp.
QSqlIndex QSqlCursor::index ( const QString & fieldName ) const
This is an overloaded member function, provided for convenience. It behaves essentially like the above function.
Returns an index based on fieldName.
QSqlIndex QSqlCursor::index ( const char * fieldName ) const
This is an overloaded member function, provided for convenience. It behaves essentially like the above function.
Returns an index based on fieldName.
void QSqlCursor::insert ( int pos, const QSqlFieldInfo & fieldInfo ) [virtual]
Insert a copy of fieldInfo at position pos. If a field
already exists at pos, it is removed. Note that all references
to the cursor edit buffer become invalidated.
Examples: sql/overview/insert/main.cpp and sql/overview/insert2/main.cpp.
int QSqlCursor::insert ( bool invalidate = TRUE ) [virtual]
This is an overloaded member function, provided for convenience. It behaves essentially like the above function.
Inserts the current contents of the cursor's edit record buffer
into the database, if the cursor allows inserts. Returns the
number of rows affected by the insert. For error information, use
lastError().
If invalidate is TRUE (the default), the cursor will no longer
be positioned on a valid record and can no longer be navigated. A
new select() call must be made before navigating to a valid
record.
QSqlCursor cur( "prices" );
QSqlRecord *buffer = cur.primeInsert();
buffer->setValue( "id", 53981 );
buffer->setValue( "name", "Thingy" );
buffer->setValue( "price", 105.75 );
cur.insert();
In the above example, a cursor is created on the 'prices' table
and a pointer to the insert buffer is aquired using primeInsert().
Each field's value is set to the desired value and then insert()
is called to insert the data into the database. Remember: all edit
operations (insert(), update() and delete()) operate on the
contents of the cursor edit buffer and not on the contents of the
cursor itself.
See also setMode() and lastError().
bool QSqlCursor::isCalculated ( const QString & name ) const
Returns TRUE if the field name exists and is calculated;
otherwise returns FALSE.
See also setCalculated().
bool QSqlCursor::isNull ( int i ) const
Returns TRUE if the field i is NULL or if there is no field at
position i; otherwise returns FALSE.
This is the same as calling QSqlRecord::isNull( i )
bool QSqlCursor::isNull ( const QString & name ) const
This is an overloaded member function, provided for convenience. It behaves essentially like the above function.
Returns TRUE if the field called name is NULL or if there is no
field called name; otherwise returns FALSE.
This is the same as calling QSqlRecord::isNull( name )
bool QSqlCursor::isReadOnly () const
Returns TRUE if the cursor is read-only; otherwise returns FALSE.
The default is FALSE. Read-only cursors cannot be edited using
insert(), update() or del().
See also setMode().
bool QSqlCursor::isTrimmed ( const QString & name ) const
Returns TRUE if the field name exists and is trimmed; otherwise
returns FALSE.
When a trimmed field of type string or cstring is read from the
database any trailing (right-most) spaces are removed.
See also setTrimmed().
int QSqlCursor::mode () const
Returns the current cursor mode.
See also setMode().
QString QSqlCursor::name () const
Returns the name of the cursor.
QSqlCursor & QSqlCursor::operator= ( const QSqlCursor & other )
Sets the cursor equal to other.
QSqlIndex QSqlCursor::primaryIndex ( bool setFromCursor = TRUE ) const [virtual]
Returns the primary index associated with the cursor as defined in
the database, or an empty index if there is no primary index. If
setFromCursor is TRUE (the default), the index fields are
populated with the corresponding values in the cursor's current
record.
QSqlRecord * QSqlCursor::primeDelete () [virtual]
This function primes the edit buffer's field values for delete and
returns the edit buffer. The default implementation copies the
field values from the current cursor record into the edit buffer
(therefore, this function is equivalent to calling editBuffer(
TRUE ) ). The cursor retains ownership of the returned pointer, so
it must not be deleted or modified.
See also editBuffer() and del().
Example: sql/overview/delete/main.cpp.
QSqlRecord * QSqlCursor::primeInsert () [virtual]
This function primes the edit buffer's field values for insert and
returns the edit buffer. The default implementation clears all
field values in the edit buffer. The cursor retains ownership of
the returned pointer, so it must not be deleted or modified.
See also editBuffer() and insert().
Examples: sql/overview/insert/main.cpp, sql/overview/insert2/main.cpp, sql/overview/subclass5/main.cpp, and sql/sqltable/main.cpp.
QSqlRecord * QSqlCursor::primeUpdate () [virtual]
This function primes the edit buffer's field values for update and
returns the edit buffer. The default implementation copies the
field values from the current cursor record into the edit buffer
(therefore, this function is equivalent to calling editBuffer(
TRUE ) ). The cursor retains ownership of the returned pointer, so
it must not be deleted or modified.
See also editBuffer() and update().
Examples: sql/overview/custom1/main.cpp, sql/overview/form1/main.cpp, and sql/overview/update/main.cpp.
void QSqlCursor::remove ( int pos ) [virtual]
Removes the field at pos. If pos does not exist, nothing
happens. Note that all references to the cursor edit buffer become
invalidated.
Reimplemented from QSqlRecord.
bool QSqlCursor::select ( const QString & filter, const QSqlIndex & sort = QSqlIndex ( ) ) [virtual]
Selects all fields in the cursor from the database matching the
filter criteria filter. The data is returned in the order
specified by the index sort. Returns TRUE if the data was
successfully selected; otherwise returns FALSE.
The filter is a string containing a SQL WHERE clause but
without the 'WHERE' keyword. The cursor is initially positioned at
an invalid row after this function is called. To move to a valid
row, use seek(), first(), last(), prev() or next().
Example:
QSqlCursor cur( "Employee" ); // Use the Employee table or view
cur.select( "deptno=10" ); // select all records in department 10
while( cur.next() ) {
... // process data
}
...
// select records in other departments, ordered by department number
cur.select( "deptno>10", cur.index( "deptno" ) );
...
The filter will apply to any subsequent select() calls that do not
explicitly specify another filter. Similarly the sort will apply
to any subsequent select() calls that do not explicitly specify
another sort.
QSqlCursor cur( "Employee" );
cur.select( "deptno=10" ); // select all records in department 10
while( cur.next() ) {
... // process data
}
...
cur.select(); // re-selects all records in department 10
...
Examples: sql/overview/delete/main.cpp, sql/overview/extract/main.cpp, sql/overview/order1/main.cpp, sql/overview/order2/main.cpp, sql/overview/retrieve2/main.cpp, sql/overview/table3/main.cpp, and sql/overview/update/main.cpp.
bool QSqlCursor::select ()
This is an overloaded member function, provided for convenience. It behaves essentially like the above function.
Selects all fields in the cursor from the database. The rows are
returned in the order specified by the last call to setSort() or
the last call to select() that specified a sort, whichever is the
most recent. If there is no current sort, the order in which the
rows are returned is undefined. The records are filtered according
to the filter specified by the last call to setFilter() or the
last call to select() that specified a filter, whichever is the
most recent. If there is no current filter, all records are
returned. The cursor is initially positioned at an invalid row. To
move to a valid row, use seek(), first(), last(), prev() or
next().
See also setSort() and setFilter().
bool QSqlCursor::select ( const QSqlIndex & sort )
This is an overloaded member function, provided for convenience. It behaves essentially like the above function.
Selects all fields in the cursor from the database. The data is
returned in the order specified by the index sort. The records
are filtered according to the filter specified by the last call to
setFilter() or the last call to select() that specified a filter,
whichever is the most recent. The cursor is initially positioned
at an invalid row. To move to a valid row, use seek(), first(),
last(), prev() or next().
bool QSqlCursor::select ( const QSqlIndex & filter, const QSqlIndex & sort )
This is an overloaded member function, provided for convenience. It behaves essentially like the above function.
Selects all fields in the cursor matching the filter index filter. The data is returned in the order specified by the index
sort. The filter index works by constructing a WHERE clause
using the names of the fields from the filter and their values
from the current cursor record. The cursor is initially positioned
at an invalid row. To move to a valid row, use seek(), first(),
last(), prev() or next(). This function is useful, for example,
for retrieving data based upon a table's primary index:
QSqlCursor cur( "Employee" );
QSqlIndex pk = cur.primaryIndex();
cur.setValue( "id", 10 );
cur.select( pk, pk ); // generates "SELECT ... FROM Employee WHERE id=10 ORDER BY id"
...
In this example the QSqlIndex, pk, is used for two different
purposes. When used as the filter (first) argument, the field
names it contains are used to construct the WHERE clause, each set
to the current cursor value, WHERE id=10, in this case. When
used as the sort (second) argument the field names it contains are
used for the ORDER BY clause, ORDER BY id in this example.
void QSqlCursor::setCalculated ( const QString & name, bool calculated ) [virtual]
Sets field name to calculated. If the field name does not
exist, nothing happens. The value of a calculated field is set by
the calculateField() virtual function which you must reimplement
(or the field value will be an invalid QVariant). Calculated
fields do not appear in generated SQL statements sent to the
database.
See also calculateField() and QSqlRecord::setGenerated().
void QSqlCursor::setFilter ( const QString & filter ) [virtual]
Sets the current filter to filter. Note that no new records are
selected. To select new records, use select(). The filter will
apply to any subsequent select() calls that do not explicitly
specify a filter.
The filter is a SQL WHERE clause without the keyword 'WHERE',
e.g. name='Dave' which will be processed by the DBMS.
void QSqlCursor::setGenerated ( const QString & name, bool generated ) [virtual]
Sets the generated flag for the field name to generated. If
the field does not exist, nothing happens. Only fields that have
generated set to TRUE are included in the SQL that is
generated by insert(), update() or del().
See also isGenerated().
Reimplemented from QSqlRecord.
void QSqlCursor::setGenerated ( int i, bool generated ) [virtual]
This is an overloaded member function, provided for convenience. It behaves essentially like the above function.
Sets the generated flag for the field i to generated.
See also isGenerated().
Reimplemented from QSqlRecord.
void QSqlCursor::setMode ( int mode ) [virtual]
Sets the cursor mode to mode. This value can be an OR'ed
combination of QSqlCursor::Mode values. The default mode for a
cursor is QSqlCursor::Writable.
QSqlCursor cur( "Employee" );
cur.setMode( QSqlCursor::Writable ); // allow insert/update/delete
...
cur.setMode( QSqlCursor::Insert | QSqlCursor::Update ); // allow inserts and updates only
...
cur.setMode( QSqlCursor::ReadOnly ); // no inserts/updates/deletes allowed
void QSqlCursor::setName ( const QString & name, bool autopopulate = TRUE ) [virtual]
Sets the name of the cursor to name. If autopopulate is TRUE
(the default), the name must correspond to a valid table or
view name in the database. Also, note that all references to the
cursor edit buffer become invalidated when fields are
auto-populated. See the QSqlCursor constructor documentation for
more information.
void QSqlCursor::setPrimaryIndex ( const QSqlIndex & idx ) [virtual]
Sets the primary index associated with the cursor to the index idx. Note that this index must contain a field or set of fields
which identify a unique record within the underlying database
table or view so that update() and del() will execute as expected.
See also update() and del().
void QSqlCursor::setSort ( const QSqlIndex & sort ) [virtual]
Sets the current sort to sort. Note that no new records are
selected. To select new records, use select(). The sort will
apply to any subsequent select() calls that do not explicitly
specify a sort.
void QSqlCursor::setTrimmed ( const QString & name, bool trim ) [virtual]
Sets field name's trimmed status to trim. If the field name does not exist, nothing happens.
When a trimmed field of type string or cstring is read from the
database any trailing (right-most) spaces are removed.
See also isTrimmed() and QVariant.
QSqlIndex QSqlCursor::sort () const
Returns the current sort, or an empty index if there is no current
sort.
QString QSqlCursor::toString ( QSqlRecord * rec, const QString & prefix, const QString & fieldSep, const QString & sep ) const [virtual protected]
Returns a formatted string composed of all the fields in rec.
Each field is composed of the prefix (e.g. table or view name),
".", the field name, the fieldSep and the field value. If the
prefix is empty then each field will begin with the field name.
The fields are then joined together separated by sep. Fields
where isGenerated() returns FALSE are not included. This function
is useful for generating SQL statements.
QString QSqlCursor::toString ( const QString & prefix, QSqlField * field, const QString & fieldSep ) const [virtual protected]
This is an overloaded member function, provided for convenience. It behaves essentially like the above function.
Returns a formatted string composed of the prefix (e.g. table
or view name), ".", the field name, the fieldSep and the
field value. If the prefix is empty then the string will begin
with the field name. This function is useful for generating SQL
statements.
QString QSqlCursor::toString ( const QSqlIndex & i, QSqlRecord * rec, const QString & prefix, const QString & fieldSep, const QString & sep ) const [virtual protected]
This is an overloaded member function, provided for convenience. It behaves essentially like the above function.
Returns a formatted string composed of all the fields in the index
i. Each field is composed of the prefix (e.g. table or view
name), ".", the field name, the fieldSep and the field value.
If the prefix is empty then each field will begin with the field
name. The field values are taken from rec. The fields are then
joined together separated by sep. Fields where isGenerated()
returns FALSE are ignored. This function is useful for generating
SQL statements.
int QSqlCursor::update ( bool invalidate = TRUE ) [virtual]
Updates the database with the current contents of the edit buffer.
Returns the number of records which were updated.
For error information, use lastError().
Only records which meet the filter criteria specified by the
cursor's primary index are updated. If the cursor does not contain
a primary index, no update is performed and 0 is returned.
If invalidate is TRUE (the default), the current cursor can no
longer be navigated. A new select() call must be made before you
can move to a valid record. For example:
QSqlCursor cur( "prices" );
cur.select( "id=202" );
if ( cur.next() ) {
QSqlRecord *buffer = cur.primeUpdate();
double price = buffer->value( "price" ).toDouble();
double newprice = price * 1.05;
buffer->setValue( "price", newprice );
cur.update();
}
In the above example, a cursor is created on the 'prices' table
and is positioned on the record to be updated. Then a pointer to
the cursor's edit buffer is acquired using primeUpdate(). A new
value is calculated and placed into the edit buffer with the
setValue() call. Finally, an update() call is made on the cursor
which uses the tables's primary index to update the record in the
database with the contents of the cursor's edit buffer. Remember:
all edit operations (insert(), update() and delete()) operate on
the contents of the cursor edit buffer and not on the contents of
the cursor itself.
Note that if the primary index does not uniquely distinguish
records the database may be changed into an inconsistent state.
See also setMode() and lastError().
Example: sql/overview/update/main.cpp.
int QSqlCursor::update ( const QString & filter, bool invalidate = TRUE ) [virtual protected]
This is an overloaded member function, provided for convenience. It behaves essentially like the above function.
Updates the database with the current contents of the cursor edit
buffer using the specified filter. Returns the number of
records which were updated.
For error information, use lastError().
Only records which meet the filter criteria are updated, otherwise
all records in the table are updated.
If invalidate is TRUE (the default), the cursor can no longer
be navigated. A new select() call must be made before you can move
to a valid record.
See also primeUpdate(), setMode(), and lastError().
This file is part of the Qt toolkit.
Copyright © 1995-2005
Trolltech. All Rights Reserved.
