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) a2 12 2select (select a from t3), a from t2;(select a from t3) aNULL 1NULL 2select * from t2 where t2.a=(select a from t1);a b2 7insert into t3 values (6),(7),(3);select * from t2 where t2.b=(select a from t3 order by 1 limit 1);a b1 72 7select * 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 b1 72 73 8drop table t1,t2,t3,t4;