predicates are based on logical universal and existential quantifiers that are recognised in formal logic. In subquery expressions, quantified predicates make it possible to compare separate values with unequal to, every element of the right-side stream. Nevertheless, it aligns perfectly with formal logic: if the set is empty, the predicate is true 0 times, i.e. for every row in the set. ANY and SOME column that does not have the NOT NULL constraint. For UNIQUE keys that span multiple columns, the logic is a little complicated: • Multiple rows having null in all the columns of the key are allowed •

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 3.3. More logic (or not) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . NULL “A and A” always equals A — which is NULL. All these results are in accordance with boolean logic. The fact that you don’t need to know X's value to compute “X or true” and “X and false” is also rules which operation, look at the second letter: tRue prevails with oR — fAlse with And. 3.3. More logic (or not) The short-circuit results obtained above may lead you to the following ideas: Chapter 3

predicates are based on logical universal and existential quantifiers that are recognised in formal logic. In subquery expressions, quantified predicates make it possible to compare separate values with unequal to, every element of the right-side stream. Nevertheless, it aligns perfectly with formal logic: if the set is empty, the predicate is true 0 times, i.e., for every row in the set. ANY and SOME column that does not have the NOT NULL constraint. For UNIQUE keys that span multiple columns, the logic is a little complicated: • Multiple rows having null in all the columns of the key are allowed •