Safety of Expressions

next up previous
Next: Expressive Power of Up: The Tuple Relational Previous: Formal Definitions

Safety of Expressions

  1. A tuple relational calculus expression may generate an infinite expression, e.g.

  2. There are an infinite number of tuples that are not in borrow! Most of these tuples contain values that do not appear in the database.

  3. Safe Tuple Expressions

    We need to restrict the relational calculus a bit.

    We may say an expression is safe if all values that appear in the result are values from dom().

  4. A safe expression yields a finite number of tuples as its result. Otherwise, it is called unsafe.

Page created and maintained by Osmar R. Zaï ane
Last Update: Wed Sep 20 15:45:57 PDT 1995