# 简单表达式
#### 逻辑表达式
逻辑表达式的操作符和运算规则，请参见[逻辑操作符](https://support.huaweicloud.com/distributed-devg-v2-gaussdb/gaussdb-12-0247.html#ZH-CN_TOPIC_0000002187528244)。
#### 比较表达式
常用的比较操作符，请参见[比较操作符](https://support.huaweicloud.com/distributed-devg-v2-gaussdb/gaussdb-12-0248.html#ZH-CN_TOPIC_0000002222774273)。
除比较操作符外，还可以使用以下句式结构：
- BETWEEN操作符 a BETWEEN x AND y等效于a \>= x AND a \<= y
  a NOT BETWEEN x AND y等效于a \< x OR a \> y
  
- 检查一个值是不是null，可使用： expressionIS NULL
  expression IS NOT NULL
  或者与之等价的句式结构，但不是标准的：
  expression ISNULL
  expression NOTNULL
  ![](https://support.huaweicloud.com/distributed-devg-v2-gaussdb/public_sys-resources/notice_3.0-zh-cn.png)
  不要写expression=NULL或expression\<\>(!=)NULL，因为NULL代表一个未知的值，不能通过该表达式判断两个未知值是否相等。
  
 
#### 示例
```
openGauss=# SELECT 2 BETWEEN 1 AND 3 AS RESULT;
 result 
----------
 t
(1 row)
openGauss=# SELECT 2 >= 1 AND 2 <= 3 AS RESULT;
 result 
----------
 t
(1 row)
openGauss=# SELECT 2 NOT BETWEEN 1 AND 3 AS RESULT;
 result 
----------
 f
(1 row)
openGauss=# SELECT 2 < 1 OR 2 > 3 AS RESULT;
 result 
----------
 f
(1 row)
openGauss=# SELECT 2+2 IS NULL AS RESULT;
 result 
----------
 f
(1 row)
openGauss=# SELECT 2+2 IS NOT NULL AS RESULT;
 result 
----------
 t
(1 row)
openGauss=# SELECT 2+2 ISNULL AS RESULT;
 result 
----------
 f
(1 row)
openGauss=# SELECT 2+2 NOTNULL AS RESULT;
 result 
----------
 t
(1 row)
openGauss=# SELECT 2+2 IS DISTINCT FROM NULL AS RESULT;
 result 
----------
 t
(1 row)
openGauss=# SELECT 2+2 IS NOT DISTINCT FROM NULL AS RESULT;
 result  
----------
 f
(1 row)
```