Constraint attributes specify integrity rules. Constraints can be any of the following
types:
Uniqueness (see “Uniqueness Constraints” on page 31).
CHECK (see “CHECK Constraints” on page 31).
Referential integrity (see “Referential Constraints” on page 32).
You can specify constraints during table creation and modification.
Column constraints apply to single columns as a part of the column definition. Column
constraints include:
CHECK constraint definition clause on a single column
PRIMARY KEY constraint definition clause on a single column
REFERENCES constraint definition clause on a single column
UNIQUE constraint definition clause
Table constraints apply to multiple columns. Table-level constraints include:
CHECK constraint definition clause on multiple columns
REFERENCES constraint definition clause on multiple columns
PRIMARY KEY constraint definition clause on multiple columns
UNIQUE constraint definition clause on multiple columns
FOREIGN KEY constraint definition clause
FOREIGN KEY constraint definitions must also specify a REFERENCES clause.
The full syntax for constraints is in “ALTER TABLE” and “CREATE TABLE” in SQL Data Definition Language.