Constraints

Constraints are rules enforced by the database manager.

DB2^® UDB for iSeries™ supports the following constraints:

• Unique constraints

  A unique constraint is the rule that the values of the key are valid only if they are unique. Unique constraints can be created using the CREATE TABLE and ALTER TABLE statements. Although CREATE INDEX can create a unique index that also guarantees uniqueness, such an index is not a constraint.

  Unique constraints are enforced during the execution of INSERT and UPDATE statements. A PRIMARY KEY constraint is a form of UNIQUE constraint. The difference is that a PRIMARY KEY cannot contain any nullable columns.

• Referential constraints

  A referential constraint is the rule that the values of the foreign key are valid only if:

  • They appear as values of a parent key, or
  • Some component of the foreign key is null.

  Referential constraints are enforced during the execution of INSERT, UPDATE, and DELETE statements.

• Check constraints

  A check constraint is a rule that limits the values allowed in a column or group of columns. Check constraints can be added using the CREATE TABLE and ALTER TABLE statements. Check constraints are enforced during the execution of INSERT and UPDATE statements. To satisfy the constraint, each row of data inserted or updated in the table must make the specified condition either TRUE or unknown (due to a null value).