DELETE grandchildren - use WHERE EXISTS or JOIN?

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DELETE grandchildren - use WHERE EXISTS or JOIN?

Maxer

SSCrazy Eights

Points: 8964
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February 2, 2010 at 12:21 pm

#386233

So someone pointed out an interesting discussion they ran across and now I was wondering about it:
DELETE grandchild
FROM grandchild gc
JOIN child c on c.child_id=gc.child_id
join parent p on p.parent = c.parent_id
vs.
DELETE grandchild gc
WHERE EXISTS
(SELECT *
FROM child c
JOIN parent p
ON c.parent_id = p.parent_id
WHERE gc.child_id = c.child_id
)
-crappy formatting there I know
Anyway, why would you use EXISTS over the more direct join?

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John Rowan SSC Guru Points: 56440 More actions · Answer 1

Your question is one that is often asked and debated. Check out this link:

http://www.sqlservercentral.com/Forums/Topic848551-360-1.aspx?Highlight=%22my+first+article%22

John Rowan

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Gail Shaw SSC Guru Points: 1004504 More actions · Answer 2

Because the join isn't necessary. All you're trying to do is find rows that exist in the other table. So why not use EXISTS?

One of these may have been the 'interesting discussion'

http://sqlinthewild.co.za/index.php/2010/01/12/in-vs-inner-join/

http://sqlinthewild.co.za/index.php/2009/08/17/exists-vs-in/

Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability

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