Left join vs Exists

Question

Left join vs Exists

mihir.nasikkar 40164

SSC Rookie

Points: 28
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October 1, 2015 at 5:40 am

#305116

SELECT DISTINCT mv.meetID,
mv.FundID,
CASE WHEN vcv.vote IS NULL
THEN mv.VoteFlag
ELSE (CASE WHEN vcv.vote IN ('Unknown','Refer',' ','Did Not Vote','None') Then 0 Else 1 END) END AS VoteFlag
FROM dbo.Information mv
LEFT OUTER JOIN dbo.InformationOverride vcv ON vcv.meetID = mv.meetID AND vcv.FundID = mv.FundID AND vcv.agendaID = mv.agendaID
Can this query be written using exists clause ?
The information table has close to 4.5 million records . MeetID has clustered index and there non clustered on fundID.
There is composite clustered index on InformationOverride table on MeetID, FundID and AgendaId.

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ChrisM@Work SSC Guru Points: 186127 More actions · Answer 1

mihir.nasikkar 40164 (10/1/2015)
SELECT DISTINCT mv.meetID, mv.FundID, CASE WHEN vcv.vote IS NULL THEN mv.VoteFlag ELSE (CASE WHEN vcv.vote IN ('<Unknown>','Refer','','Did Not Vote','None') Then 0 Else 1 END) END AS VoteFlag FROM dbo.Information mv LEFT OUTER JOIN dbo.InformationOverride vcv ON vcv.meetID = mv.meetID AND vcv.FundID = mv.FundID AND vcv.agendaID = mv.agendaID

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ScottPletcher SSC Guru Points: 101126 More actions · Answer 2

No, you can't use EXISTS to return data.

As to performance, the non-clustered index on the vcv (InformationOverride) table would have to INCLUDE the vote column to be useful.

Better would very likely be to cluster the vcv table first on meetID; you may want/need to add FundID and agendaID, can't tell from this query alone.

SQL DBA,SQL Server MVP(07, 08, 09) "It's a dog-eat-dog world, and I'm wearing Milk-Bone underwear." "Norm", on "Cheers". Also from "Cheers", from "Carla": "You need to know 3 things about Tortelli men: Tortelli men draw women like flies; Tortelli men treat women like flies; Tortelli men's brains are in their flies".

Grey Cat Mr or Mrs. 500 Points: 524 More actions · Answer 3

Not sure what you are looking for...if just want to get rid of left outer join, a sub-query for table InformationOverride would work. If record does not exist, null is returned.

myzus2004 Old Hand Points: 378 More actions · Answer 4

Thanks scott for your feedback. Appreciate it .

The clustered index on InformationOverride table cannnot be changed as the composite index on these 3 columns have unique data.

I am trying to optimize this specific code block hence trying out if i can write using exists clause.

Gail Shaw SSC Guru Points: 1004504 More actions · Answer 5

The difference between EXISTS and JOIN is minimal, so even if you could, there wouldn't be much point in doing so.

Also, a clustered index doesn't have to be unique, it's just a strong guideline that it is.

What nonclustered indexes exist on the tables? Can you post an execution plan?

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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ScottPletcher SSC Guru Points: 101126 More actions · Answer 6

myzus2004 (10/3/2015)
Thanks scott for your feedback. Appreciate it .
The clustered index on InformationOverride table cannnot be changed as the composite index on these 3 columns have unique data.
I am trying to optimize this specific code block hence trying out if i can write using exists clause.

Ah, you've changed your original q to now indicate that the vcv table is clustered by meetID, where as it originally said something different.

In that case, yes, my advice is mute, since it's already been done :-D.

SQL DBA,SQL Server MVP(07, 08, 09) "It's a dog-eat-dog world, and I'm wearing Milk-Bone underwear." "Norm", on "Cheers". Also from "Cheers", from "Carla": "You need to know 3 things about Tortelli men: Tortelli men draw women like flies; Tortelli men treat women like flies; Tortelli men's brains are in their flies".

Eric M Russell SSC Guru Points: 125562 More actions · Answer 7

It seems to me that, if InformationOverride is meant for exclusion, then you're missing a "WHERE vcf.MeetID IS NULL" clause, or if InformationOverride is meant for inclusion, then it should be an INNER JOIN instead of a LEFT (OUTER) JOIN.

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