CHECK Column Constraint
Column constraint to validate a change to 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 CHECK column constraint is used to validate any changes made to the value of a column. The <condition> specified must evaluate to True (.T.) for the modification operation to succeed. If the <condition> evaluates to False (.F.) the column value remains unchanged and the ERROR column constraint message is displayed. If the ERROR column constraint has not been defined, a default error message is displayed.
ALTER TABLE customer; ADD COLUMN timeref CHAR(8); CHECK validtime(timeref); ERROR "Not a valid time string"