ANSI Semantics with NULL and NOT IN

Given table "r1": a1 a2 a3 1 A null 2 B 1 3 C 1 On Ingres, this query returns the value "A" select a2 from r1 where a1 in (select distinct a3 from r1); However, this query returns no rows select a2 from r1 where a1 not in (select distinct a3 from r1); Where one may expect two rows ("B" and "C") The Ingres explanation is that "this is ANSI semantics when NULL is involved in the subselect, NOT IN is equivalent to !=ALL which means the values must satisfy "!=" for all values in the subselect, since, for example, 3 != NULL is FALSE given ANSI semantics, the qualification is FALSE for all rows so zero rows are returned." Is this true? What do Sybase, Oracle, Informix do? Thanks, Irwin irwin@Binky.COM