drop table if exists t1,t2,t3,t4;
create table t1 (a int);
create table t2 (a int, b int);
create table t3 (a int);
create table t4 (a int, b int);
insert into t1 values (2);
insert into t2 values (1,7),(2,7);
insert into t4 values (4,8),(3,8),(5,9);
select (select a from t1), a from t2;
(select a from t1)	a
2	1
2	2
select (select a from t3), a from t2;
(select a from t3)	a
NULL	1
NULL	2
select * from t2 where t2.a=(select a from t1);
a	b
2	7
insert into t3 values (6),(7),(3);
select * from t2 where t2.b=(select a from t3 order by 1 limit 1);
a	b
1	7
2	7
select * from t2 where t2.b=(select a from t3 order by 1 limit 1) 
union (select * from t4 order by a limit 2) limit 3;
a	b
1	7
2	7
3	8
drop table t1,t2,t3,t4;