ITPEC FE Morning April 2019 Question 27

Referential constraint — FOREIGN KEY with REFERENCES ties a column to the key of another table.

A declaration such as FOREIGN KEY (DNO) REFERENCES DEPT (DNO) says that every non-null DNO in this table must exist as a DNO in DEPT. The DBMS then rejects an insert or update that names a department that does not exist, and a delete of a department that rows still refer to, unless a rule such as ON DELETE CASCADE says otherwise. This keeps referential integrity between the two tables.

Answer (d)

Why not others:

• •(a) Assertion — a general condition over the database, declared with CREATE ASSERTION, not with FOREIGN KEY
• •(b) Check constraint — a condition on the values of a row within one table, such as CHECK (Salary > 0)
• •(c) Key constraint — PRIMARY KEY or UNIQUE, which makes values unique within one table

Key rule: Uniqueness within a table is a key constraint, a condition on values is a check constraint, and a link between tables is a referential constraint.

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