mysql中exists注意点

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执行sql:
select 1 from dual where exists (select 0);

select 1 from dual where exists (select null);

执行结果都是1；

mysql> create table t3(id int,t datetime);
Query OK, 0 rows affected (0.44 sec)

mysql> insert into t3 values(1,'20160812');

Query OK, 1 row affected (0.16 sec)

mysql> select 1 from dual where  exists (select id from t3 where id=2);
Empty set (0.15 sec)

mysql> select 1 from dual where  exists (select max(id) from t3 where id=2);
+---+
| 1 |
+---+

| 1 |

mysql> select 1 from dual where  exists (select count(id) from t3 where id=2);
+---+
| 1 |
+---+
| 1 |

mysql> select id from t3 where id=2;

Empty set (0.15 sec)

mysql> select max(id) from t3 where id=2
+---+
| NULL |
+---+

| NULL|

mysql> select count(id) from t3 where id=2
+---+
| 0 |
+---+
| 0 |

由此可见：exists里子查询执行结果是null或者0，只要有一行结果，则exist就生效执行出结果，只有子查询执行结果是Empty的时候exists才会过滤数据。