For example:<code>
create table One (A integer);
create table Two (A integer, B text);
insert into One values (100);
insert into One values (101);
insert into One values (102);
insert into Two values (100, 'X');
insert into Two values (100, 'Y');
insert into Two values (100, 'Z');
insert into Two values (101, 'Y');
insert into Two values (101, 'Z');
select A from One except select A from Two where Two.B in ('X');
</code>