SQL provides four types of integrity constraints:

• Check constraints:
  • Column-level check constraint
  • Table-level check constraint
• Primary key specifications
• Candidate key specifications

SQL allows you to specify an integrity constraint, and to refer to that constraint in other SQL statements. The database assigns a constraint name if you do not specify one.

The following example shows the assignment of table constraint prim_constr on table supplier_item. You specify a constraint name with the CONSTRAINT keyword.

 CREATE TABLE supplier_item (
          supp_no  INTEGER NOT NULL,
          item_no  INTEGER NOT NULL,
          qty      INTEGER NOT NULL DEFAULT 0
          CONSTRAINT prim_constr
          PRIMARY KEY (supp_no, item_no)
     ) ;