Comparisons Using Subqueries

Description

The most common use of a subquery is in the form:

1. non_subquery_operand comparison_operator (subquery)

Where comparison_operator is one of these operators:

1. =  >  <  >=  <=  <>  !=  <=>

Syntax

1. > SELECT column_name(s) FROM table_name WHERE 'a' = (SELECT column1 FROM t1)

Examples

1. create table t1 (a int);
2. create table t2 (a int, b int);
3. create table t3 (a int);
4. create table t4 (a int not null, b int not null);
5. insert into t1 values (2);
6. insert into t2 values (1,7),(2,7);
7. insert into t4 values (4,8),(3,8),(5,9);
8. insert into t3 values (6),(7),(3);

10. mysql> select * from t3 where a = (select b from t2);
11. ERROR 1105 (HY000): scalar subquery returns more than 1 row
12. mysql> select * from t3 where a = (select distinct b from t2);
13. +------+
14. | a    |
15. +------+
16. |    7 |
17. +------+
18. 1 rows in set (0.01 sec)

20. mysql> select a,b from t4 where a > ( select a ,b from t2 where a>1);
21. ERROR 1105 (HY000): Internal error: Unknow type TUPLE