SQL 处理顺序

Question

假设我有一个类似

的查询

select * from A
Except 
select * from B 
union all
select * from B 
except
select * from A

查询的处理方式与

相同

select * 
from 
(
select * from A
Except 
select * from B 
) a
union all
(
select * from B
Except 
select * from A 
) b

sql中定义的处理顺序是怎样的？无论如何都会这样处理

select * from A
Except 
select * from
(
select * from B 
union all
select * from B 
) a
except
select * from A

Answer 1

处理 EXCEPT 和 UNION "left to right"。这意味着没有任何括号来确定它们将按照它们在 sql 中出现的顺序进行处理。

https://msdn.microsoft.com/en-us/library/ms188055.aspx

Answer 2

UNION 和 EXCEPT 具有 相同的优先级 但将从左到右绑定，这意味着它们按“从左到右”的顺序进行评估，如他们被处理了。

来自@SeanLange 的 URL (TL;DR)，值得注意的是：

If EXCEPT or INTERSECT is used together with other operators in an expression, it is evaluated in the context of the following precedence:

Expressions in parentheses

The INTERSECT operator

EXCEPT and UNION evaluated from left to right based on their position in the expression

SQL 处理顺序

SQL Order of processing

sql

sql-server

except

union-all