Q3SqlCursor Class Reference
^{[Qt3Support module]}

Detailed Description

The Q3SqlCursor class provides browsing and editing of SQL tables and views.

A Q3SqlCursor 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(), previous(), 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:

    Q3SqlCursor 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:

    Q3SqlCursor 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.

Q3SqlCursor 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. Q3SqlCursor generates SQL statements which are sent to the database engine; you can control which fields are included in these statements using setGenerated().

Note that Q3SqlCursor does not inherit from QObject. This means that you are responsible for destroying instances of this class yourself. However if you create a Q3SqlCursor and use it in a Q3DataTable, Q3DataBrowser or a Q3DataView these classes will usually take ownership of the cursor and destroy it when they don't need it anymore. The documentation for Q3DataTable, Q3DataBrowser and Q3DataView explicitly states which calls take ownership of the cursor.

Member Type Documentation

enum Q3SqlCursor::Mode

This enum type describes how Q3SqlCursor operates on records in the database.

Member Function Documentation

Q3SqlCursor::Q3SqlCursor ( const QString & name = QString(), bool autopopulate = true, QSqlDatabase db = QSqlDatabase() )

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 Q3SqlCursor::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().

Q3SqlCursor::Q3SqlCursor ( const Q3SqlCursor & other )

Constructs a copy of other.

Q3SqlCursor::~Q3SqlCursor ()   [virtual]

Destroys the object and frees any allocated resources.

void Q3SqlCursor::append ( const Q3SqlFieldInfo & 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 Q3SqlCursor::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().

bool Q3SqlCursor::canDelete () const

Returns true if the cursor will perform deletes; otherwise returns false.

See also setMode().

bool Q3SqlCursor::canInsert () const

Returns true if the cursor will perform inserts; otherwise returns false.

See also setMode().

bool Q3SqlCursor::canUpdate () const

Returns true if the cursor will perform updates; otherwise returns false.

See also setMode().

void Q3SqlCursor::clear ()   [virtual]

Removes all fields from the cursor. Note that all references to the cursor edit buffer become invalidated.

int Q3SqlCursor::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:

    Q3SqlCursor 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().

int Q3SqlCursor::del ( const QString & filter, bool invalidate = true )   [virtual protected]

This is an overloaded member function, provided for convenience.

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 * Q3SqlCursor::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().

bool Q3SqlCursor::exec ( const QString & sql )   [virtual protected]

Executes the SQL query sql. Returns true of the cursor is active, otherwise returns false.

QString Q3SqlCursor::filter () const

Returns the current filter, or an empty string if there is no current filter.

See also setFilter().

QSqlIndex Q3SqlCursor::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.

QSqlIndex Q3SqlCursor::index ( const QString & fieldName ) const

This is an overloaded member function, provided for convenience.

Returns an index based on fieldName.

void Q3SqlCursor::insert ( int pos, const Q3SqlFieldInfo & 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.

int Q3SqlCursor::insert ( bool invalidate = true )   [virtual]

This is an overloaded member function, provided for convenience.

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.

    Q3SqlCursor 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 Q3SqlCursor::isCalculated ( const QString & name ) const

Returns true if the field name exists and is calculated; otherwise returns false.

See also setCalculated().

bool Q3SqlCursor::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 Q3SqlCursor::isNull ( const QString & name ) const

This is an overloaded member function, provided for convenience.

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 Q3SqlCursor::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 Q3SqlCursor::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 Q3SqlCursor::mode () const

Returns the current cursor mode.

See also setMode().

QString Q3SqlCursor::name () const

Returns the name of the cursor.

See also setName().

QSqlIndex Q3SqlCursor::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.

See also setPrimaryIndex().

QSqlRecord * Q3SqlCursor::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().

QSqlRecord * Q3SqlCursor::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().

QSqlRecord * Q3SqlCursor::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().

void Q3SqlCursor::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.

bool Q3SqlCursor::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(), previous() or next().

Example:

    Q3SqlCursor 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.

    Q3SqlCursor 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
    ...

bool Q3SqlCursor::select ()

This is an overloaded member function, provided for convenience.

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(), previous() or next().

See also setSort() and setFilter().

bool Q3SqlCursor::select ( const QSqlIndex & sort )

This is an overloaded member function, provided for convenience.

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(), previous() or next().

bool Q3SqlCursor::select ( const QSqlIndex & filter, const QSqlIndex & sort )

This is an overloaded member function, provided for convenience.

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(), previous() or next(). This function is useful, for example, for retrieving data based upon a table's primary index:

    Q3SqlCursor 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 Q3SqlCursor::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 isCalculated() and calculateField().

void Q3SqlCursor::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.

See also filter().

void Q3SqlCursor::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().

void Q3SqlCursor::setGenerated ( int i, bool generated )   [virtual]

This is an overloaded member function, provided for convenience.

Sets the generated flag for the field i to generated.

void Q3SqlCursor::setMode ( int mode )   [virtual]

Sets the cursor mode to mode. This value can be an OR'ed combination of Q3SqlCursor::Mode values. The default mode for a cursor is Q3SqlCursor::Writable.

    Q3SqlCursor cur("Employee");
    cur.setMode(Q3SqlCursor::Writable); // allow insert/update/delete
    ...
    cur.setMode(Q3SqlCursor::Insert | Q3SqlCursor::Update); // allow inserts and updates only
    ...
    cur.setMode(Q3SqlCursor::ReadOnly); // no inserts/updates/deletes allowed

See also mode().

void Q3SqlCursor::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 Q3SqlCursor constructor documentation for more information.

See also name().

void Q3SqlCursor::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 primaryIndex(), update(), and del().

void Q3SqlCursor::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.

See also sort().

void Q3SqlCursor::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 is read from the database any trailing (right-most) spaces are removed.

See also isTrimmed() and QVariant.

void Q3SqlCursor::setValue ( const QString & name, const QVariant & val )

This is an overloaded member function, provided for convenience.

Sets the value for the field named name to val.

QSqlIndex Q3SqlCursor::sort () const

Returns the current sort, or an empty index if there is no current sort.

See also setSort().

QString Q3SqlCursor::toString ( QSqlRecord * rec, const QString & prefix, const QString & fieldSep, const QString & sep ) const   [virtual protected]

This is an overloaded member function, provided for convenience.

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 Q3SqlCursor::toString ( const QString & prefix, QSqlField * field, const QString & fieldSep ) const   [virtual protected]

This is an overloaded member function, provided for convenience.

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 Q3SqlCursor::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.

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 Q3SqlCursor::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:

    Q3SqlCursor 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().

int Q3SqlCursor::update ( const QString & filter, bool invalidate = true )   [virtual protected]

This is an overloaded member function, provided for convenience.

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().

QVariant Q3SqlCursor::value ( int i ) const

Returns the value of field number i.

QVariant Q3SqlCursor::value ( const QString & name ) const

This is an overloaded member function, provided for convenience.

Returns the value of the field named name.

Q3SqlCursor & Q3SqlCursor::operator= ( const Q3SqlCursor & other )

Sets the cursor equal to other.