SQL and Relational Theory, 2nd Edition

CHAPTER 8

8.1 A type constraint is a definition of the set of values that constitute a given type. The type constraint for type T is checked whenever some selector for type T is invoked; if the check fails, the selector invocation fails on a type constraint violation. Subsidiary exercise: What do you think should happen if the type constraint for type T evaluates to FALSE at the time type T is defined? (Answer: This state of affairs isn’t necessarily an error, but the type in question will be empty. See the answer to Exercise 2.18 elsewhere in this appendix.)

A database constraint is a constraint on the values that can appear in a given database. Database constraints are checked “at semicolons”—more specifically, at the end of any statement that assigns a value to any of the pertinent relvars. If the check fails, the assignment fails on a database constraint violation. Note: Database constraints must also be checked when they’re defined. If that check fails, the constraint definition must be rejected.

8.2 The Golden Rule states (in effect) that no update operation must ever cause any database constraint to evaluate to FALSE, and hence a fortiori that no update operation must ever cause any relvar constraint to evaluate to FALSE either. However, a (total) relvar constraint might evaluate to FALSE, not because some single relvar constraint is violated, but rather because some multirelvar constraint is violated. The point is hardly significant, however, given that—as mentioned in the ...