mysql将null看做最大_MySQL NULL 和 NOT NULL 你知道多少

前言：

谈起这个话题，是因为想整理一个开发规范，中间涉及到一条，建表的时候建议字段定义为NOT NULL，但是为什么不可以是NULL呢？怎么让开发相信NOT NULL一定比NULL好呢，往下看把

1、NULL 和 ‘’ 值有什么区别？

NULL值会占一个到两个字节的空间，而‘’值是不会占用空间的。

https://dev.mysql.com/doc/refman/5.7/en/innodb-physical-record.html

2、为什么字段类型是 NOT NULL 可以插入 ‘’ 值呢？

看看官方给的例子和解释吧，感觉这个解释很完美

3、如何判断字段为 NULL 和 ‘’ ？

4、为什么索引中 ‘’ 比 NULL 效率高呢？

下面来看一个例子：因为B树索引是不会存储NULL值的，索引字段可以NULL，索引效率会下降很多

root@localhost:mysql.sock 15:03:25 [tom]>show create table a;

+-------+-----------------------------------------------------------------------------------------------+

| Table | Create Table |

+-------+-----------------------------------------------------------------------------------------------+

| a | CREATE TABLE `a` (

`a` int(11) DEFAULT NULL,

`b` int(11) DEFAULT NULL,

KEY `k` (`b`)

) ENGINE=InnoDB DEFAULT CHARSET=utf8 |

+-------+-----------------------------------------------------------------------------------------------+

1 row in set (0.00 sec)

root@localhost:mysql.sock 15:04:34 [tom]>SELECT * FROM a;

+------+------+

| a | b |

+------+------+

| 1 | NULL |

| 2 | NULL |

| 3 | 3 |

| 4 | 44 |

| 5 | 55 |

+------+------+

5 rows in set (0.00 sec)

root@localhost:mysql.sock 15:05:47 [tom]>explain select b from a where b is null;

+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+--------------------------+

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

+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+--------------------------+

| 1 | SIMPLE | a | NULL | ref | k | k | 5 | const | 2 | 100.00 | Using where; Using index |

+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+--------------------------+

1 row in set, 1 warning (0.00 sec)

root@localhost:mysql.sock 16:42:51 [tom]>explain select b from a where b='';

+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+-------------+

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

+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+-------------+

| 1 | SIMPLE | a | NULL | ref | k | k | 5 | const | 1 | 100.00 | Using index |

+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+-------------+

1 row in set, 1 warning (0.00 sec)

5、NULL值得其他问题

参考文档：

https://dev.mysql.com/doc/refman/5.7/en/problems-with-null.html

https://dev.mysql.com/doc/refman/5.7/en/working-with-null.html

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