Subqueries with EXISTS or NOT EXISTS
Description
The EXISTS
operator is used to test for the existence of any record in a subquery.
If a subquery returns any rows at all, EXISTS
subquery is TRUE
, and NOT EXISTS
subquery is FALSE
.
Syntax
> SELECT column_name(s)
FROM table_name
WHERE EXISTS
(SELECT column_name FROM table_name WHERE condition);
Examples
create table t1 (a int);
create table t2 (a int, b int);
create table t3 (a int);
create table t4 (a int not null, b int not null);
insert into t1 values (2);
insert into t2 values (1,7),(2,7);
insert into t4 values (4,8),(3,8),(5,9);
insert into t3 values (6),(7),(3);
mysql> select * from t3 where exists (select * from t2 where t2.b=t3.a);
+------+
| a |
+------+
| 7 |
+------+
1 row in set (0.00 sec)
mysql> select * from t3 where not exists (select * from t2 where t2.b=t3.a);
+------+
| a |
+------+
| 6 |
| 3 |
+------+
2 rows in set (0.00 sec)