create table foo (x char(10) primary key);
insert into foo values ('a_b');
insert into foo values ('ac_b');
insert into foo values ('aa_b');
insert into foo values ('aab');
insert into foo values ('a_ab');
insert into foo values ('a_a');
insert into foo values ('b_a');
insert into foo values ('ba');
insert into foo values ('b__a');
select * from foo where x = 'a_ab';
