Constraints

Constraints set rules on what data is allowed in table columns. Using constraints can help maintain data integrity. For example, you can constrain a column to allow only unique values, or to disallow NULL values. Constraints such as primary keys also help the optimizer generate query plans that facilitate faster data access, particularly for joins.

You set constraints on a new table and an existing one with CREATE TABLE and ALTER TABLE...ADD CONSTRAINT, respectively.

To view current constraints, see Column-constraint.