Mysql高手系列 - 第13篇：细说NULL致使的神坑，让人防不胜防

时间 2020-05-05 标签 mysql 高手系列细说 null 致使让人防不胜防

这是Mysql系列第13篇。html

环境：mysql5.7.25，cmd命令中进行演示。java

当数据的值为NULL的时候，可能出现各类意想不到的效果，让人防不胜防，咱们来看看NULL致使的各类神坑，如何避免？mysql

比较运算符中使用NULL

认真看下面的效果sql

mysql> select 1>NULL;
+--------+
| 1>NULL |
+--------+
|   NULL |
+--------+
1 row in set (0.00 sec)

mysql> select 1<NULL;
+--------+
| 1<NULL |
+--------+
|   NULL |
+--------+
1 row in set (0.00 sec)

mysql> select 1<>NULL;
+---------+
| 1<>NULL |
+---------+
|    NULL |
+---------+
1 row in set (0.00 sec)

mysql> select 1>NULL;
+--------+
| 1>NULL |
+--------+
|   NULL |
+--------+
1 row in set (0.00 sec)

mysql> select 1<NULL;
+--------+
| 1<NULL |
+--------+
|   NULL |
+--------+
1 row in set (0.00 sec)

mysql> select 1>=NULL;
+---------+
| 1>=NULL |
+---------+
|    NULL |
+---------+
1 row in set (0.00 sec)

mysql> select 1<=NULL;
+---------+
| 1<=NULL |
+---------+
|    NULL |
+---------+
1 row in set (0.00 sec)

mysql> select 1!=NULL;
+---------+
| 1!=NULL |
+---------+
|    NULL |
+---------+
1 row in set (0.00 sec)

mysql> select 1<>NULL;
+---------+
| 1<>NULL |
+---------+
|    NULL |
+---------+
1 row in set (0.00 sec)
    
mysql> select NULL=NULL,NULL!=NULL;
+-----------+------------+
| NULL=NULL | NULL!=NULL |
+-----------+------------+
|      NULL |       NULL |
+-----------+------------+
1 row in set (0.00 sec)
    
mysql> select 1 in (null),1 not in (null),null in (null),null not in (null);
+-------------+-----------------+----------------+--------------------+
| 1 in (null) | 1 not in (null) | null in (null) | null not in (null) |
+-------------+-----------------+----------------+--------------------+
|        NULL |            NULL |           NULL |               NULL |
+-------------+-----------------+----------------+--------------------+
1 row in set (0.00 sec)

mysql> select 1=any(select null),null=any(select null);
+--------------------+-----------------------+
| 1=any(select null) | null=any(select null) |
+--------------------+-----------------------+
|               NULL |                  NULL |
+--------------------+-----------------------+
1 row in set (0.00 sec)
    
mysql> select 1=all(select null),null=all(select null);
+--------------------+-----------------------+
| 1=all(select null) | null=all(select null) |
+--------------------+-----------------------+
|               NULL |                  NULL |
+--------------------+-----------------------+
1 row in set (0.00 sec)

结论：任何值和NULL使用运算符（>、<、>=、<=、!=、<>）或者（in、not in、any/some、all）比较时，返回值都为NULL，NULL做为布尔值的时候，不为1也不为0。微信

准备数据

mysql> create table test1(a int,b int);
Query OK, 0 rows affected (0.01 sec)

mysql> insert into test1 values (1,1),(1,null),(null,null);
Query OK, 3 rows affected (0.00 sec)
Records: 3  Duplicates: 0  Warnings: 0

mysql> select * from test1;
+------+------+
| a    | b    |
+------+------+
|    1 |    1 |
|    1 | NULL |
| NULL | NULL |
+------+------+
3 rows in set (0.00 sec)

上面3条数据，认真看一下，特别是注意上面NULL的记录。函数

IN、NOT IN和NULL比较

IN和NULL比较

mysql> select * from test1;
+------+------+
| a    | b    |
+------+------+
|    1 |    1 |
|    1 | NULL |
| NULL | NULL |
+------+------+
3 rows in set (0.00 sec)

mysql> select * from test1 where a in (null);
Empty set (0.00 sec)

mysql> select * from test1 where a in (null,1);
+------+------+
| a    | b    |
+------+------+
|    1 |    1 |
|    1 | NULL |
+------+------+
2 rows in set (0.00 sec)

结论：当IN和NULL比较时，没法查询出为NULL的记录。spa

NOT IN 和NULL比较

mysql> select * from test1 where a not in (1);
Empty set (0.00 sec)

mysql> select * from test1 where a not in (null);
Empty set (0.00 sec)

mysql> select * from test1 where a not in (null,2);
Empty set (0.00 sec)
    
mysql> select * from test1 where a not in (2);
+------+------+
| a    | b    |
+------+------+
|    1 |    1 |
|    1 | NULL |
+------+------+
2 rows in set (0.00 sec)

结论：当NOT IN 后面有NULL值时，不论什么状况下，整个sql的查询结果都为空。3d

EXISTS、NOT EXISTS和NULL比较

mysql> select * from test2;
+------+------+
| a    | b    |
+------+------+
|    1 |    1 |
|    1 | NULL |
| NULL | NULL |
+------+------+
3 rows in set (0.00 sec)

mysql> select * from test1 t1 where exists (select * from test2 t2 where t1.a = t2.a);
+------+------+
| a    | b    |
+------+------+
|    1 |    1 |
|    1 | NULL |
+------+------+
2 rows in set (0.00 sec)

mysql> select * from test1 t1 where not exists (select * from test2 t2 where t1.a = t2.a);
+------+------+
| a    | b    |
+------+------+
| NULL | NULL |
+------+------+
1 row in set (0.00 sec)

上面咱们复制了表test1建立了表test2。code

查询语句中使用exists、not exists对比test1.a=test2.a，由于=不能比较NULL，结果和预期一致。htm

判断NULL只能用IS NULL、IS NOT NULL

mysql> select 1 is not null;
+---------------+
| 1 is not null |
+---------------+
|             1 |
+---------------+
1 row in set (0.00 sec)

mysql> select 1 is null;
+-----------+
| 1 is null |
+-----------+
|         0 |
+-----------+
1 row in set (0.00 sec)

mysql> select null is null;
+--------------+
| null is null |
+--------------+
|            1 |
+--------------+
1 row in set (0.00 sec)

mysql> select null is not null;
+------------------+
| null is not null |
+------------------+
|                0 |
+------------------+
1 row in set (0.00 sec)

看上面的效果，返回的结果为1或者0。

结论：判断是否为空只能用IS NULL、IS NOT NULL。

聚合函数中NULL的坑

示例

mysql> select count(a),count(b),count(*) from test1;
+----------+----------+----------+
| count(a) | count(b) | count(*) |
+----------+----------+----------+
|        2 |        1 |        3 |
+----------+----------+----------+
1 row in set (0.00 sec)

count(a)返回了2行记录，a字段为NULL的没有统计出来。

count(b)返回了1行记录，为NULL的2行记录没有统计出来。

count(*)能够统计全部数据，不论字段的数据是否为NULL。

再继续看

mysql> select * from test1 where a is null;
+------+------+
| a    | b    |
+------+------+
| NULL | NULL |
+------+------+
1 row in set (0.00 sec)
    
mysql> select count(a) from test1 where a is null;
+----------+
| count(a) |
+----------+
|        0 |
+----------+
1 row in set (0.00 sec)

上面第1个sql使用is null查询出告终果，第2个sql中count(a)返回的是0行。

结论：count(字段)没法统计字段为NULL的值，count(*)能够统计值为null的行。

NULL不能做为主键的值

mysql> create table test3(a int primary key,b int);
Query OK, 0 rows affected (0.01 sec)

mysql> insert into test3 values (null,1);
ERROR 1048 (23000): Column 'a' cannot be null

上面咱们建立了一个表test3，字段a未指定不能为空，插入了一条NULL的数据，报错缘由：a 字段的值不能为NULL，咱们看一下表的建立语句：

mysql> show create table test3;
+-------+------------+
| Table | Create Table      |
+-------+------------+
| test3 | CREATE TABLE `test3` (
  `a` int(11) NOT NULL,
  `b` int(11) DEFAULT NULL,
  PRIMARY KEY (`a`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8
+-------+------------+
1 row in set (0.00 sec)

从上面的脚本能够看出，当字段为主键的时候，字段会自动设置为not null。

结论：当字段为主键的时候，字段会自动设置为not null。

看了上面这些仍是比较晕，NULL的状况确实比较难以处理，容易出错，最有效的方法就是避免使用NULL。因此，强烈建议建立字段的时候字段不容许为NULL，设置一个默认值。

总结

NULL做为布尔值的时候，不为1也不为0
任何值和NULL使用运算符（>、<、>=、<=、!=、<>）或者（in、not in、any/some、all），返回值都为NULL
当IN和NULL比较时，没法查询出为NULL的记录
当NOT IN 后面有NULL值时，不论什么状况下，整个sql的查询结果都为空
判断是否为空只能用IS NULL、IS NOT NULL
count(字段)没法统计字段为NULL的值，count(*)能够统计值为null的行
当字段为主键的时候，字段会自动设置为not null
NULL致使的坑让人防不胜防，强烈建议建立字段的时候字段不容许为NULL，给个默认值

Mysql系列目录

<a target="_blank" href="https://mp.weixin.qq.com/s?__biz=MzA5MTkxMDQ4MQ==&mid=2648933257&idx=1&sn=0f0086a2465a2fcae13d3fea65064803&chksm=88621bb7bf1592a1ac94fe4107ba1ef26a0fa97e1bf9aea7279009d8bd240f1ef7d27aa10393&token=1876080189&lang=zh_CN#rd">第1篇：mysql基础知识</a>
<a target="_blank" href="https://mp.weixin.qq.com/s?__biz=MzA5MTkxMDQ4MQ==&mid=2648933270&idx=1&sn=409080e17352da2035b0bfdf63ccdfde&chksm=88621ba8bf1592beb2ef6106d6bf9f3eccd48d6814c7031f36e3c8be68821f17cf065129688c&token=1876080189&lang=zh_CN#rd"> 第2篇：详解mysql数据类型（重点）</a>
<a target="_blank" href="https://mp.weixin.qq.com/s?__biz=MzA5MTkxMDQ4MQ==&mid=2648933279&idx=1&sn=f8591b95362cb3c352d895b1289d665a&chksm=88621ba1bf1592b72a43a62e3f310695e8b87f17932d052145622c3edbb70ef8cb987849fc3e&token=516655478&lang=zh_CN#rd"> 第3篇：管理员必备技能(必须掌握)</a>
<a target="_blank" href="https://mp.weixin.qq.com/s?__biz=MzA5MTkxMDQ4MQ==&mid=2648933289&idx=1&sn=c4f212c312ea86e08ad322caddd05e38&chksm=88621b97bf159281156ee3be510a1a15234531d2c97d66957e67377829ab23779809ea55bbde&token=1484565200&lang=zh_CN#rd"> 第4篇：DDL常见操做</a>
<a target="_blank" href="https://mp.weixin.qq.com/s?__biz=MzA5MTkxMDQ4MQ==&mid=2648933296&idx=1&sn=1c56256d60c5847a944d87c8cfc9c14d&chksm=88621b8ebf159298b0789e2994d2aaf8b582effc7d8c1ba715deaca11c86a9dc8ac730878dc0&token=2000571846&lang=zh_CN#rd"> 第5篇：DML操做汇总（insert,update,delete）</a>
<a target="_blank" href="https://mp.weixin.qq.com/s?__biz=MzA5MTkxMDQ4MQ==&mid=2648933300&idx=1&sn=bedef4d430dc76141e42e42ef6acfaa6&chksm=88621b8abf15929caae7904019c946a396885a33855ca465bacdd4187538005ebc3c116888f5&token=1814800041&lang=zh_CN#rd"> 第6篇：select查询基础篇</a>
<a target="_blank" href="https://mp.weixin.qq.com/s?__biz=MzA5MTkxMDQ4MQ==&mid=2648933305&idx=1&sn=2c5ba0bea6fcdd57d86cecd63541f91a&chksm=88621b87bf1592915086c1e945119fcc95af6aa1127b90ef1b56b018083bfd787ad95efde918&token=1144227002&lang=zh_CN#rd"> 第7篇：玩转select条件查询，避免采坑</a>
<a target="_blank" href="https://mp.weixin.qq.com/s?__biz=MzA5MTkxMDQ4MQ==&mid=2648933309&idx=1&sn=0f7dfec8bc70e67daa7159ee219325b8&chksm=88621b83bf1592951d949179061f39a1f4266b2879bc9a25af4da3b4b29f69ab1fcc595a462f&token=516674265&lang=zh_CN#rd"> 第8篇：详解排序和分页(order by & limit)</a>
<a target="_blank" href="https://mp.weixin.qq.com/s?__biz=MzA5MTkxMDQ4MQ==&mid=2648933315&idx=1&sn=4abf2b34a53a85cbd4a3b9ee31fcd398&chksm=88621bfdbf1592eb6a5a13202588dde9068ac7e6391d8dbf3fde33647bf0e044839ba4228216&token=1937580929&lang=zh_CN#rd"> 第9篇：分组查询详解（group by & having）</a>
<a target="_blank" href="https://mp.weixin.qq.com/s?__biz=MzA5MTkxMDQ4MQ==&mid=2648933321&idx=1&sn=110f6cacb90845bf2327fbcd7acd708c&chksm=88621bf7bf1592e1f1c0f2f01e40d4bd63a48d98daa4ba1157ecad4c6c6520d18b4b2c24c906&token=1096041061&lang=zh_CN#rd"> 第10篇：经常使用的几十个函数详解</a>
<a target="_blank" href="https://mp.weixin.qq.com/s?__biz=MzA5MTkxMDQ4MQ==&mid=2648933325&idx=1&sn=33274227db275a3570e1e43ccdd4f49c&chksm=88621bf3bf1592e5b75f537e21961c4295fafb782dbf31c0c1cbf9e36f5f4ed44d31cdad68b3&token=1832579722&lang=zh_CN#rd"> 第11篇：深刻了解链接查询及原理</a>
<a target="_blank" href="https://mp.weixin.qq.com/s?__biz=MzA5MTkxMDQ4MQ==&mid=2648933342&idx=2&sn=2e41daa0926a9c32d5fddd23590391aa&chksm=88621be0bf1592f66904a184858df7c11129e8b545c7626b8b6c1fcc32d3dfdd9ab21aeda8e5&token=649921610&lang=zh_CN#rd"> 第12篇：子查询（很是重要，高手必备）</a>

mysql系列大概有20多篇，喜欢的请关注一下，欢迎你们加我微信itsoku或者留言交流mysql相关技术!

原文出处：https://www.cnblogs.com/itsoku123/p/11582628.html