Subqueries with EXISTS or NOT EXISTS
语法描述
EXISTS
用于检查子查询是否至少会返回一行数据。即将主查询的数据,放到子查询中做条件验证,根据验证结果(TRUE 或 FALSE)来决定主查询的数据结果是否得以保留。
如果子查询返回任何行,EXISTS
子查询条件为 TRUE
,NOT EXISTS
子查询条件为 FALSE
。
语法结构
> SELECT column_name(s)
FROM table_name
WHERE EXISTS
(SELECT column_name FROM table_name WHERE condition);
示例
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)