Short-Circuit Evaluation
When computing a logical expression, the PL/SQL uses short-circuit evaluation. That is, the PL/SQL stops computing the expression as soon as the result can be determined. This permits you to write the expressions that might otherwise cause an error. Now consider the following OR expression:
DECLARE
...
on_hand INTEGER;
on_order INTEGER;
BEGIN
..
IF (on_hand = 0) OR ((on_order / on_hand) < 5) THEN
...
END IF;
END;
Whenever the value of on_hand is zero, the left operand yields TRUE, so the PL/SQL require not compute the right operand. If PL/SQL were to compute both operands before applying the OR operator, the right operand would cause a division by the zero error. In any case, it is a poor programming practice to rely on the short-circuit computation.