Subquery

Question

Subquery

SQLSACT

SSC-Insane

Points: 21838
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November 9, 2012 at 2:16 pm

#393576

Hi guys
Trying to gear up my T-SQL skills
When dealing with subqueries, like below
select Col1 from Tbl1 where Col1 = '52' and Col1 in (select Col2 from Tbl2)
What is the order of the execution here? Does SQL first check for the value 52 in Tbl2.Col2 and then check for the value 52 in Tbl1.Col1
Thanks

Viewing 9 posts - 1 through 8 (of 8 total)

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Roland Alexander STL SSCrazy Points: 2915 More actions · Answer 1

Maybe, maybe not. Depends on many factors such as indexing, clustering, number of rows in the tables, etc. Check the execution plan to see what's getting processed when. Might surprise you!

Roland Alexander
The Monday Morning DBA
There are two means of refuge from the miseries of life: music and cats. ~ Albert Schweitzer

SQLSACT SSC-Insane Points: 21838 More actions · Answer 2

Roland Alexander STL (11/9/2012)
Maybe, maybe not. Depends on many factors such as indexing, clustering, number of rows in the tables, etc. Check the execution plan to see what's getting processed when. Might surprise you!

Thanks for the reply

I want to be able to completely understand what is fundamentally happening here

The subquery has its results and the outer query has its own results as well, are these 2 results cross referenced?

Thanks

Sean Lange SSC Guru Points: 286590 More actions · Answer 3

I would write that query a little differently. I would instead write it as

select t1.Col1

from Tbl1 t1

join Tbl2 t2 on t1.Col1 = t2.Col2

where t1.Col1 = '52'

That to me is a LOT easier to read and understand quickly. Check out the execution plans for both queries. It will really surprise you!!!

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SQLSACT SSC-Insane Points: 21838 More actions · Answer 4

Sean Lange (11/9/2012)
I would write that query a little differently. I would instead write it as
select t1.Col1
from Tbl1 t1
join Tbl2 t2 on t1.Col1 = t2.Col2
where t1.Col1 = '52'
That to me is a LOT easier to read and understand quickly. Check out the execution plans for both queries. It will really surprise you!!!

Thanks Sean

I'm just trying to better understand T-SQL

When dealing with correlated subqueries, are these 2 queries essentially the same?

--1

select T1.Col1 from T1

where exists

(select Col1 from T2 where T2.Col1 = T1.Col1)

--2

select T1.Col1 from T1

where T1.Col1 in

(select Col1 from T2 where T2.Col1 = T1.Col1)

Thanks

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

Well, those aren't written correctly. The correct form would be

--1

select T1.Col1 from T1

where exists

(select * from T2 where T2.Col1 = T1.Col1)

--2

select T1.Col1 from T1

where T1.Col1 in

(select T2.Col1 from T2)

And those two are completely equivalent.

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

We walk in the dark places no others will enter
We stand on the bridge and no one may pass

SQLSACT SSC-Insane Points: 21838 More actions · Answer 6

GilaMonster (11/12/2012)
Well, those aren't written correctly. The correct form would be
--1
select T1.Col1 from T1
where exists
(select * from T2 where T2.Col1 = T1.Col1)
--2
select T1.Col1 from T1
where T1.Col1 in
(select Col1 from T2)
And those two are completely equivalent.
http://sqlinthewild.co.za/index.php/2009/08/17/exists-vs-in/

Thanks Gail

I think I'm getting there

When using in adding a correlation predicate is redundant?

In my own words, using the query below. The subquery has already satisfied what I'm looking therefore rendering the outer query unnecessary.

Something like select Col1 from table1 where table1 in (select Col1 from table1 )

Am I on track here?

select T1.Col1 from T1

where T1.Col1 in

(select Col1 from T2 where T2.Col1 = T1.Col1)

Thanks

Gail Shaw SSC Guru Points: 1004504 More actions · Answer 7

SQLSACT (11/12/2012)
When using in adding a correlation predicate is redundant?

Written correctly, just about all the time. Please read that blog post and maybe BoL on how the IN function works

Am I on track here?
select T1.Col1 from T1
where T1.Col1 in
(select Col1 from T2 where T2.Col1 = T1.Col1)

No, I gave you the correct form in my previous post.

--2
select T1.Col1 from T1
where T1.Col1 in (select T2.Col1 from T2)

Think about what the IN does with the two columns specified and you should understand why the correlation is completely unnecessary.

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

We walk in the dark places no others will enter
We stand on the bridge and no one may pass

SQLSACT SSC-Insane Points: 21838 More actions · Answer 8

GilaMonster (11/12/2012)
SQLSACT (11/12/2012)
When using in adding a correlation predicate is redundant?
Written correctly, just about all the time. Please read that blog post and maybe BoL on how the IN function works
Am I on track here?
select T1.Col1 from T1
where T1.Col1 in
(select Col1 from T2 where T2.Col1 = T1.Col1)
No, I gave you the correct form in my previous post.
--2
select T1.Col1 from T1
where T1.Col1 in (select Col1 from T2)
Think about what the IN does with the two columns specified and you should understand why the correlation is completely unnecessary.

Thanks

I see now why it's unnecessary

When I asked "am I on track here" followed by the query. I was just confirming that that query was incorrect based on my Layman's terms.

Thanks