mysql中find_in_set的用法

create table tmp_b(a int,b int);
create table tmp_a(a varchar(24),b int);
insert into tmp_b select 1,13;
insert into tmp_b select 2,13;
insert into tmp_a select '1,2',13;
select *from tmp_a as a ,tmp_b as b  where find_in_set(b.a,a.a) and b.a=1;