MySQL 5.7 NOT EXISTS的用法是什么

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在MySQL中没有MINUS语句，要想实现MINUS语句，可以使用NOT EXISTS语句。

查看测试表中的数据
mysql> select * from person;

| MSISDN | IMEI | IMSI |

| +3301000000000 | 129980000000000 | 208011000000000 |
| +3301000000001 | 129980000000100 | 208011000000001 |
| +3301000000002 | 129980000000200 | 208011000000002 |
| +3301000000003 | 129980000000300 | 208011000000003 |
| +3301000000004 | 129980000000400 | 208011000000004 |
| +3301000000005 | 129980000000500 | 208011000000005 |
| +3301000000006 | 129980000000600 | 208011000000006 |
| +3301000000007 | 129980000000700 | 208011000000007 |

8 rows in set (0.00 sec)
mysql> select * from card;

| IMSI | MSISDN | IMEI |

| 208011000000000 | +3301000000000 | 129980000000000 |
| 208011000000001 | +3301000000001 | 129980000000100 |
| 208011000000002 | +3301000000002 | 129980000000200 |
| 208011000000003 | +3301000000003 | 129980000000300 |
| 208011000000004 | +3301000000004 | 129980000000400 |
| 208011000000005 | +3301000000005 | 129980000000500 |
| 208011000000006 | +3301000000006 | 129980000000600 |
| 208011000000007 | +3301000000007 | 129980000000700 |
| 208011000000008 | +3301000000008 | 129980000000800 |
| 208011000000009 | +3301000000009 | 129980000000900 |

10 rows in set (0.00 sec)

查询出在card中存在，但是在person中不存在的某一列MSISDN。

mysql> select * from card a where not exists(select * from person b where a.MSISDN=b.MSISDN);

| IMSI | MSISDN | IMEI |

| 208011000000008 | +3301000000008 | 129980000000800 |
| 208011000000009 | +3301000000009 | 129980000000900 |

2 rows in set (0.00 sec)

查看执行计划
mysql> explain select * from card a where not exists(select * from person b where a.MSISDN=b.MSISDN);

| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |

| 1 | PRIMARY | a | NULL | ALL | NULL | NULL | NULL | NULL | 10 | 100.00 | Using where |
| 2 | DEPENDENT SUBQUERY | b | NULL | eq_ref | PRIMARY | PRIMARY | 20 | fire.a.MSISDN | 1 | 100.00 | Using where; Using index |

2 rows in set, 2 warnings (0.00 sec)

通过执行计划，card表使用了全表扫描，person表使用了主键索引
mysql> show keys from card;

| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |

| card | 0 | PRIMARY | 1 | IMSI | A | 12 | NULL | NULL | | BTREE | | |

6 rows in set (0.00 sec)
mysql> show keys from person;

| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |

| person | 0 | PRIMARY | 1 | MSISDN | A | 8 | NULL | NULL | | BTREE | | |

1 row in set (0.00 sec)

由于两张表的主键不是相同列，创建索引进行测试，发现card表始终无法使用索引扫描
mysql> create index idx_card_msisdn on card(msisdn);
Query OK, 0 rows affected (0.22 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> explain select * from card a where not exists(select * from person b where a.MSISDN=b.MSISDN);

| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |

| 1 | PRIMARY | a | NULL | ALL | NULL | NULL | NULL | NULL | 10 | 100.00 | Using where |
| 2 | DEPENDENT SUBQUERY | b | NULL | eq_ref | PRIMARY | PRIMARY | 20 | fire.a.MSISDN | 1 | 100.00 | Using where; Using index |

2 rows in set, 2 warnings (0.00 sec)

mysql> create index idx_card_msisdn_imsi on card(msisdn,imsi);
Query OK, 0 rows affected (0.06 sec)
Records: 0 Duplicates: 0 Warnings: 0

mysql> explain select * from card a where not exists(select * from person b where a.MSISDN=b.MSISDN);

| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |

| 1 | PRIMARY | a | NULL | ALL | NULL | NULL | NULL | NULL | 10 | 100.00 | Using where |
| 2 | DEPENDENT SUBQUERY | b | NULL | eq_ref | PRIMARY | PRIMARY | 20 | fire.a.MSISDN | 1 | 100.00 | Using where; Using index |

2 rows in set, 2 warnings (0.00 sec)

mysql> create index idx_card_imsi_msisdn on card(imsi,msisdn);
Query OK, 0 rows affected (0.06 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> explain select * from card a where not exists(select * from person b where a.MSISDN=b.MSISDN);

| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+--------------------+-------+------------+--------+---------------+---------+---------+---------------+------+----------+--------------------------+
| 1 | PRIMARY | a | NULL | ALL | NULL | NULL | NULL | NULL | 10 | 100.00 | Using where |
| 2 | DEPENDENT SUBQUERY | b | NULL | eq_ref | PRIMARY | PRIMARY | 20 | fire.a.MSISDN | 1 | 100.00 | Using where; Using index |
+----+--------------------+-------+------------+--------+---------------+---------+---------+---------------+------+----------+--------------------------+
2 rows in set, 2 warnings (0.00 sec)

mysql> explain select * from card a where not exists(select * from person b where a.MSISDN=b.MSISDN);
+----+--------------------+-------+------------+--------+---------------+---------+---------+---------------+------+----------+--------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |

| 1 | PRIMARY | a | NULL | ALL | NULL | NULL | NULL | NULL | 10 | 100.00 | Using where |
| 2 | DEPENDENT SUBQUERY | b | NULL | eq_ref | PRIMARY | PRIMARY | 20 | fire.a.MSISDN | 1 | 100.00 | Using where; Using index |

2 rows in set, 2 warnings (0.00 sec)

以上就是MySQL 5.7 NOT EXISTS的用法是什么，小编相信有部分知识点可能是我们日常工作会见到或用到的。希望你能通过这篇文章学到更多知识。更多详情敬请关注行业资讯频道。