SQL Order of processing

Question

Consider I have a query like

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

Query is processed like

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

How is the order of processing defined in sql. Will it process like this at any case

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

score 5 · Answer 1 · answered Jan 19 '17 at 17:07

5

EXCEPT and UNION are processed "left to right". Meaning that without any parenthesis to make the determination they will process in the order they appear in the sql.

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

answered Jan 19 '17 at 17:07

Sean Lange

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score 3 · Accepted Answer · edited Jun 20 '20 at 09:12

UNION and EXCEPT have equal precedence but will bind from left to right, meaning they are evaluated in "left to right" order, as they are processed.

From @SeanLange's URL (TL;DR), worth taking note of:

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 Order of processing

2 Answers2