mysql空和null的区别

https://blog.youkuaiyun.com/yu757371316/article/details/53033118

 https://blog.youkuaiyun.com/u014743697/article/details/54136092/

 https://blog.youkuaiyun.com/qq_15096707/article/details/51541274

 https://www.cnblogs.com/wangzhongqiu/p/6424712.html

mysql> create table if not exists `test`(`a` varchar(10) not null,`b` varchar(10) null);
Query OK, 0 rows affected (0.56 sec)

mysql> desc test;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| a     | varchar(10) | NO   |     | NULL    |       |
| b     | varchar(10) | YES  |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.12 sec)
mysql> insert into `test` values (null,'1');
1048 - Column 'a' cannot be null //a设置为Not Null后无法插入Null值，但可插入'' 。
mysql> insert into `test`(`a`,`b`) values (1,null);
Query OK, 1 row affected (0.07 sec)

mysql> select*from `test`;
+---+------+
| a | b    |
+---+------+
| 1 | NULL |
+---+------+
1 row in set (0.03 sec)

mysql> insert into `test` values('','');//空字符可正常插入。
Query OK, 1 row affected (0.07 sec)

mysql> select*from `test`;
+---+------+
| a | b    |
+---+------+
| 1 | NULL |
|   |      |
+---+------+
2 rows in set (0.04 sec)

mysql> insert into `test` values('',null);
Query OK, 1 row affected (0.03 sec)

mysql> select*from `test`;
+---+------+
| a | b    |
+---+------+
| 1 | NULL |
|   |      |
|   | NULL |
+---+------+
3 rows in set (0.05 sec)

mysql> insert into `test` values(1,2);
Query OK, 1 row affected (0.04 sec)

mysql> select*from `test`;
+---+------+
| a | b    |
+---+------+
| 1 | NULL |
|   |      |
|   | NULL |
| 1 | 2    |
+---+------+
4 rows in set (0.03 sec)

mysql> select*from `test` where `a` is not null; //证明''与 Not Null是相对比较独立的两个东西。
+---+------+
| a | b    |
+---+------+
| 1 | NULL |
|   |      |
|   | NULL |
| 1 | 2    |
+---+------+
4 rows in set (0.05 sec)

mysql> select*from `test` where `b` is not null;
+---+---+
| a | b |
+---+---+
|   |   |
| 1 | 2 |
+---+---+
2 rows in set (0.04 sec)

mysql> select*from `test` where `a`<>''; 
+---+------+
| a | b    |
+---+------+
| 1 | NULL |
| 1 | 2    |
+---+------+
2 rows in set (0.06 sec)

mysql> select * from `test` where `b`<>''; //<>''非空字符会将Null排除在外，此处比较奇怪。
+---+---+
| a | b |
+---+---+
| 1 | 2 |
+---+---+
1 row in set (0.03 sec)

mysql> select*from `test` where `b`=''; //选择''时，Null不算在此列，相对独立。
+---+---+
| a | b |
+---+---+
|   |   |
+---+---+
1 row in set (0.05 sec)

上述命令，大致可以看到 '' 、<>''与Null和Not Null几乎可以说是相对独立的两个东西，但是有一处比较奇怪，需要注意<>'' 会把Null排除在外，这点又会使人觉得''就是Null，具体为何，还是不知道。

另外一点奇怪的是，我用的参考文章的方法和命令，选出的结果却是不同的，不知道是不是mysql版本不同造成的，我的版本是免安装5.7.22。