Computed columns versus computed fields

When creating a DataWindow object, you can define computed columns and computed fields as follows:

• In the SQL Select painter, you can define computed columns when you are defining the SELECT statement that will be used to retrieve data into the DataWindow object.

• In the DataWindow painter, you can define computed fields after you have defined the SELECT statement (or other data source).

When you define the computed column in the SQL Select painter, the value is calculated by the DBMS when the data is retrieved. The computed column’s value does not change until data has been updated and retrieved again.

When you define the computed field in the DataWindow painter, the value of the column is calculated in the DataWindow object after the data has been retrieved. The value changes dynamically as the data in the DataWindow object changes.

Consider a DataWindow object with four columns: Part number, Quantity, Price, and Cost. Cost is computed as Quantity * Price.

If Cost is defined as a computed column in the SQL Select painter, the SELECT statement is as follows:

SELECT part.part_num,	
part.part_qty,	
part.part_price,	
part.part_qty * part.part_price
FROM part;

If the user changes the price of a part in the DataWindow object in this scenario, the cost does not change in the DataWindow object until the database is updated and the data is retrieved again. The user sees a display with the changed price but the unchanged, incorrect cost.

If Cost is defined as a computed field in the DataWindow object, the SELECT statement is as follows, with no computed column:

SELECT part.part_num,	
part.part_qty,	
part.part_price
FROM part;

The computed field is defined in the DataWindow object as Quantity * Price.

In this scenario, if the user changes the price of a part in the DataWindow object, the cost changes immediately.

If you want your DBMS to do the calculations on the server before bringing data down and you do not need the computed values to be updated dynamically, define the computed column as part of the SELECT statement.

If you need computed values to change dynamically, define computed fields in the DataWindow painter Design view, as described next.