Constraints

Constraint

A constraint is a named check in SQL. They can be defined buy some are built in. e.g UNIQUE, NOT NULL, REFERENCES. Constraints prevent users or software from corrupting data in the db.

Arbitrary checks can be defined with the CHECK keyword. For example a named constraint looks like this:

CONSTRAINT valid_discount CHECK(price > discounted_price)

In the case of REFERENCES, 3 modes can be set:

• RESTRICT: Prevents changes to a row with a reference.
• SET NULL: will set row with rerference to NULL on changes.
• CASCADE: will remove row with reference on changes.