SQL逻辑运算符优先级:And和Or

Question

以下两个陈述是否相同？

SELECT [...]
FROM [...]
WHERE some_col in (1,2,3,4,5) AND some_other_expr

和

SELECT [...]
FROM [...]
WHERE some_col in (1,2,3) or some_col in (4,5) AND some_other_expr

我可以使用某种真值表来验证这一点吗？

Answer 1

And优先于Or,即使如此a <=> a1 Or a2

Where a And b 

是不一样的

Where a1 Or a2 And b,

因为那将被执行为

Where a1 Or (a2 And b)

你想要什么,使它们变得一样,是

 Where (a1 Or a2) And b

这是一个例子来说明:

Declare @x tinyInt = 1
Declare @y tinyInt = 0
Declare @z tinyInt = 0

Select Case When @x=1 OR @y=1 And @z=1 Then 'T' Else 'F' End -- outputs T
Select Case When (@x=1 OR @y=1) And @z=1 Then 'T' Else 'F' End -- outputs F

对于那些喜欢咨询参考文献的人(按字母顺序排列):

• Microsoft Transact-SQL运算符优先级
• Oracle MySQL 9运算符优先级
• Oracle 10g条件优先
• PostgreSQL运算符Precedence
• SQLite理解的SQL

Answer 2

我会加2分:

• "IN"实际上是带有括号的串行OR
• 在我所知道的每种语言中,AND优先于OR

所以,2个表达式根本不相等.

WHERE some_col in (1,2,3,4,5) AND some_other_expr
--to the optimiser is this
WHERE
     (
     some_col = 1 OR
     some_col = 2 OR 
     some_col = 3 OR 
     some_col = 4 OR 
     some_col = 5
     )
     AND
     some_other_expr

因此,当您打破IN子句时,将串行OR分开,并更改优先级.

Answer 3

1. 算术运算符
2. 连接运算符
3. 比较条件
4. IS [NOT] NULL,LIKE,[NOT] IN
5. [NOT] BETWEEN
6. 不等于
7. 不是逻辑条件
8. 和逻辑条件
9. 或逻辑条件

您可以使用括号覆盖优先级规则.

Answer 4

查询以显示3变量布尔表达式真值表:

;WITH cteData AS
(SELECT 0 AS A, 0 AS B, 0 AS C
UNION ALL SELECT 0,0,1
UNION ALL SELECT 0,1,0
UNION ALL SELECT 0,1,1
UNION ALL SELECT 1,0,0
UNION ALL SELECT 1,0,1
UNION ALL SELECT 1,1,0
UNION ALL SELECT 1,1,1
)
SELECT cteData.*,
    CASE WHEN

(A=1) OR (B=1) AND (C=1)

    THEN 'True' ELSE 'False' END AS Result
FROM cteData

结果(A=1) OR (B=1) AND (C=1):

A   B   C   Result
0   0   0   False
0   0   1   False
0   1   0   False
0   1   1   True
1   0   0   True
1   0   1   True
1   1   0   True
1   1   1   True

结果(A=1) OR ( (B=1) AND (C=1) )是一样的.

结果( (A=1) OR (B=1) ) AND (C=1):

A   B   C   Result
0   0   0   False
0   0   1   False
0   1   0   False
0   1   1   True
1   0   0   False
1   0   1   True
1   1   0   False
1   1   1   True