CALCULATED Column Constraint
Column constraint to calculate the value of a column
A constraint is used to define rules that help to provide data integrity. Column constraints are specific to the column name specified. You must have ALTER privilege on the table. The table will be locked for EXCLUSIVE use during the operation.
The CALCULATED column constraint is used to calculate the value of a column whenever it is accessed. The <expr> specified must evaluate to the same data type as the target column. Specifying the CALCULATED column constraint causes the column to be read only. If the <expr> is based on other columns from the same table, these columns must have the RECALCULATE column constraint set, so that any changes to their values cause calculated fields to be recalculated.
ALTER TABLE customer; ALTER COLUMN available CALCULATED limit-balance; ALTER COLUMN limit RECALCULATE; ALTER COLUMN balance RECALCULATE