1、创建测试表：
drop table if exists tab_null_operator;
create table tab_null_operator as
select 1 as id,'chavin' as name union all
select 2 as id,'nope' as name union all
select 3 as id,'' as name union all
select 4 as id,'' as name union all
select 5 as id,null as name union all
select 6 as id,null as name union all
select 7 as id,' ' as name union all
select 8 as id,' ' as name union all
select 9 as id,' ' as name union all
select 10 as id,' ' as name
;
2、查看数据：
mysql> select * from tab_null_operator;
+----+--------+
| id | name |
+----+--------+
| 1 | chavin |
| 2 | nope |
| 3 | |
| 4 | |
| 5 | NULL |
| 6 | NULL |
| 7 | |
| 8 | |
| 9 | |
| 10 | |
+----+--------+
10 rows in set (0.00 sec)
小结：从结果我们可以看出，id in (5,6)的两个值是null，id in (1,2)的是字符串，id in (3,4,7,8,9,10)从结果看均是空字符串，实际id in (3,4)是空字符串，id in (7,8)是单个空格字符串，id in (9,10)是 字符串。
3、查询name为null的记录：
mysql> select * from tab_null_operator where name is null;
+----+------+
| id | name |
+----+------+
| 5 | NULL |
| 6 | NULL |
+----+------+
2 rows in set (0.00 sec)
小结：可以看到只有id in (5,6)的记录name字段才是真正的null。
4、查询name为''的记录信息：
mysql> select * from tab_null_operator where name = '';
+----+------+
| id | name |
+----+------+
| 3 | |
| 4 | |
| 7 | |
| 8 | |
+----+------+
4 rows in set (0.00 sec)
小结：可以看到我们输入的以空格为字符串的值都表现为空字符串。然后 字符串的缺没有筛选出来。
5、查询 字符串的数据：
mysql> select * from tab_null_operator where name = ' ';
+----+------+
| id | name |
+----+------+
| 9 | |
| 10 | |
+----+------+
2 rows in set (0.00 sec)