Re: Non-Relating Join Criteria



Fred. wrote:
Transact SQL apparently supports one-sided critera,
such as the "a.SEL=15" in (1) below. in the join
expression. I would expect that both (1) and (2)
would return the same result set, and the examples
I have tried match this expectation.

My question is whether or not there can be a strong
reason to prefer one over the other, such as impact
on the order of evaluation.

CREATE TABLE a (APK int PRIMARY KEY, SEL int, ... )
CREATE TABLE b (BPK int PRIMARY KEY, AFK int, ... )

(1) SELECT ... FROM a INNER JOIN b
ON a.APK=b.AFK AND a.SEL=15

(2) SELECT ... FROM a INNER JOIN b
ON a.APK=b.AFK
WHERE a.SEL=15

There is a nice article about this in BOL (Books Online - it's installed
when you install the client tools).
The short story is:
with inner joins there is no difference - the same query execution plan is
used for both variations.

With outer joins, however, different results can be obtained depending on
where you put the criterion, especially when the predicate applies to the
table on the right side of a left outer join:

(1) SELECT ... FROM b LEFT JOIN a
ON a.APK=b.AFK AND a.SEL=15

(2) SELECT ... FROM b LEFT JOIN a
ON a.APK=b.AFK
WHERE a.SEL=15


.