A constraint can also be corrective. Such a constraint undertakes corrective measures when a certain rule is violated, or performs some other action that changes the database. You define a corrective constraint as an INSERT, DELETE, or UPDATE statement.
For example, you can use a corrective constraint to ensure that a guide's discount percentage is always at least 10%, as follows:
SET discount = 10
discount < 10
discount IS NULL
AND guide = 'Y'
When you define a corrective constraint, you also may specify a message. If the constraint updates a record, the message is shown as an information message.
Alternatively, you can use a corrective constraint to keep track of the number of people who have booked a place on a tour. That is, each time a reservation is made, the corrective constraint can increment the total number of participants.