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。

比较表达式

常用的比较操作符，请参见比较操作符。

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：

  1 2	expression IS NULL expression IS NOT NULL

  或者与之等价的句式结构，但不是标准的：

  1 2	expression ISNULL expression NOTNULL

  

  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

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59

SELECT 2 BETWEEN 1 AND 3 AS RESULT;
 result 
----------
 t
(1 row)

SELECT 2 >= 1 AND 2 <= 3 AS RESULT;
 result 
----------
 t
(1 row)

SELECT 2 NOT BETWEEN 1 AND 3 AS RESULT;
 result 
----------
 f
(1 row)

SELECT 2 < 1 OR 2 > 3 AS RESULT;
 result 
----------
 f
(1 row)

SELECT 2+2 IS NULL AS RESULT;
 result 
----------
 f
(1 row)

SELECT 2+2 IS NOT NULL AS RESULT;
 result 
----------
 t
(1 row)

SELECT 2+2 ISNULL AS RESULT;
 result 
----------
 f
(1 row)

SELECT 2+2 NOTNULL AS RESULT;
 result 
----------
 t
(1 row)

SELECT 2+2 IS DISTINCT FROM NULL AS RESULT;
 result 
----------
 t
(1 row)

SELECT 2+2 IS NOT DISTINCT FROM NULL AS RESULT;
 result  
----------
 f
(1 row)