项目场景：

查看测试表test_1，发现表字段classes里面有null值，过滤null值。

--查看
> select * from test_1;
+------------+-----------------+
| test_1.id  | test_1.classes  |
+------------+-----------------+
| Mary       | class 1         |
| James      | class 2         |
| lily       | null            |
| Mike       | NULL            |
| Herry      | class 1         |
+------------+-----------------+

---

问题描述

使用where classes is null过滤没有成功。

>  select * from test_1 where classes is null;
>  select * from test_1 where classes is NULL;
>  select * from test_1 where classes is not null;
>  select * from test_1 where classes is not NULL;--运行结果：
+------------+-----------------+
| test_1.id  | test_1.classes  |
+------------+-----------------+
+------------+-----------------+

运行的结果都是为空的，并没有将classes为null或者NULL对应的id过滤出来。

---

原因分析：

使用 is null / is not null 对string类型字段进行过滤无效。

--查看表结构
> desc test_1;+-----------+------------+----------+
| col_name  | data_type  | comment  |
+-----------+------------+----------+
| id        | string     |          |
| classes   | string     |          |
+-----------+------------+----------+

可以看到classes的类型是string，hive的底层保存的是’null’、'NULL’是个字符串，想要过滤掉null或者NULL值，使用is not null无效。

---

解决方案：

对于字符串字段，使用 =‘null’，=‘NULL’，!= ‘null’，!= ‘NULL’ 进行过滤。

>  select * from test_1 where classes = 'null';
+------------+-----------------+
| test_1.id  | test_1.classes  |
+------------+-----------------+
| lily       | null            |
+------------+-----------------+>  select * from test_1 where classes = 'NULL';
+------------+-----------------+
| test_1.id  | test_1.classes  |
+------------+-----------------+
| Mike       | NULL            |
+------------+-----------------+>  select * from test_1 where classes != 'null';
+------------+-----------------+
| test_1.id  | test_1.classes  |
+------------+-----------------+
| Mary       | class 1         |
| James      | class 2         |
| Mike       | NULL            |
| Herry      | class 1         |
+------------+-----------------+>  select * from test_1 where classes != 'NULL';
+------------+-----------------+
| test_1.id  | test_1.classes  |
+------------+-----------------+
| Mary       | class 1         |
| James      | class 2         |
| lily       | null            |
| Herry      | class 1         |
+------------+-----------------+