INNER JOIN on TOP 1

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INNER JOIN on TOP 1

Simon E Doubt

SSCarpal Tunnel

Points: 4191
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June 24, 2008 at 8:10 am

#381353

Good Morning, everyone:
I'm trying to join two tables.
However, I only want to return the TOP 1 row from the child table.
As a simplified example, consider the following two tables (I've included the code to create the temp tables and data below):
TableA
=====
RetailerID
54
33
TableB
=====
RetailerID, RetailerIDTheirs
54, 'ABC'
54, 'DEF'
33, 'TUV'
33, 'WXYZ'
I would like to know how to write the JOIN syntax to return only the TOP 1 row from TableB.
The only solution that I know how to do is something like this:
SELECT
A.RetailerID,
(SELECT TOP 1 RetailerIDTheirs FROM TableB WHERE RetailerID = A.RetailerID ORDER BY RetailerIDTheirs ASC) AS RetailerIDTheirs
FROM TableA AS A
Which returns exactly what I want:
This works fine, except that when I try to group the data, I get an error message telling me:
"Cannot use an aggregate or a subquery in an expression used for the group by list of a GROUP BY clause."
For the purpose of keeping things simple, I've left-out the part of the code that includes the grouping.
As always, thanks to everyone for any thoughts/input.
-Simon
Code for creating sample tables and rows:
-- Create TableA, the Parent table
CREATE TABLE TableA
(RetailerID INT NOT NULL PRIMARY KEY CLUSTERED)
-- Populate TableA
INSERT TableA (RetailerID)
VALUES (54)
INSERT TableA (RetailerID)
VALUES (33)
-- Create TableB, the Child table
CREATE TABLE TableB
(
RetailerID INT NOT NULL
CONSTRAINT FK_TableB_RetailerID FOREIGN KEY ([RetailerID]) REFERENCES TableA ([RetailerID]),
RetailerIDTheirs VARCHAR(10) NOT NULL
)
-- Populate TableB
INSERT TableB (RetailerID, RetailerIDTheirs)
VALUES (54, 'ABC')
INSERT TableB (RetailerID, RetailerIDTheirs)
VALUES (54, 'DEF')
INSERT TableB (RetailerID, RetailerIDTheirs)
VALUES (33, 'TUV')
INSERT TableB (RetailerID, RetailerIDTheirs)
VALUES (33, 'WXYZ')

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

Hi Simon

This is simple but somewhat restrictive:

SELECT A.RetailerID, MIN(B.RetailerIDTheirs) AS RetailerIDTheirs

FROM #TableA A

LEFT JOIN #TableB B ON B.RetailerID = A.RetailerID

GROUP BY A.RetailerID

This is less simple but offers more flexibility:

SELECT A.RetailerID, B2.*

FROM #TableA A

LEFT JOIN (SELECT RetailerID, MIN(RetailerIDTheirs) AS RetailerIDTheirs FROM #TableB GROUP BY RetailerID) B ON B.RetailerID = A.RetailerID

LEFT JOIN #TableB B2 ON B2.RetailerID = B.RetailerID AND B2.RetailerIDTheirs = B.RetailerIDTheirs

Both are likely to be more efficient than a correlated subquery.

^{“Write the query the simplest way. If through testing it becomes clear that the performance is inadequate, consider alternative query forms.” - Gail Shaw}

For fast, accurate and documented assistance in answering your questions, please read this article.
Understanding and using APPLY, (I) and (II) Paul White
Hidden RBAR: Triangular Joins / The "Numbers" or "Tally" Table: What it is and how it replaces a loop Jeff Moden

Mark Cowne One Orange Chip Points: 26944 More actions · Answer 2

Another alternative

SELECT

A.RetailerID,

X.RetailerIDTheirs

FROM TableA AS A

CROSS APPLY (SELECT TOP 1 B.RetailerIDTheirs FROM TableB B WHERE B.RetailerID = A.RetailerID ORDER BY B.RetailerIDTheirs ASC) AS X

____________________________________________________

Deja View - The strange feeling that somewhere, sometime you've optimised this query before

How to get the best help on a forum

http://www.sqlservercentral.com/articles/Best+Practices/61537

Simon E Doubt SSCarpal Tunnel Points: 4191 More actions · Answer 3

Mark and Chris,

Thank you so much for your replies.

I'll try both and let you know what works best.

Thanks again - I really appreciate it.

-Simon

ChrisM@Work SSC Guru Points: 186127 More actions · Answer 4

Mark (6/24/2008)
Another alternative
SELECT
A.RetailerID,
X.RetailerIDTheirs
FROM TableA AS A
CROSS APPLY (SELECT TOP 1 B.RetailerIDTheirs FROM TableB B WHERE B.RetailerID = A.RetailerID ORDER BY B.RetailerIDTheirs ASC) AS X

Nice one, Mark, and far closer to the OP's requirements. We're still on 2k here.

^{“Write the query the simplest way. If through testing it becomes clear that the performance is inadequate, consider alternative query forms.” - Gail Shaw}

For fast, accurate and documented assistance in answering your questions, please read this article.
Understanding and using APPLY, (I) and (II) Paul White
Hidden RBAR: Triangular Joins / The "Numbers" or "Tally" Table: What it is and how it replaces a loop Jeff Moden

Grant Fritchey SSC Guru Points: 398679 More actions · Answer 5

This will work with 2000:

SELECT

A.RetailerID,

b.RetailerIDTheirs

FROM TableA AS A

LEFT JOIN TableB AS B

ON A.RetailerID = B.RetailerID

AND b.RetailerIDTheirs = (SELECT TOP 1 B2.RetailerIDTheirs

FROM TableB B2

WHERE B2.RetailerID = A.RetailerID

ORDER BY B2.RetailerIDTheirs ASC)

"The credit belongs to the man who is actually in the arena, whose face is marred by dust and sweat and blood"
- Theodore Roosevelt

Author of:
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SQL Server Query Performance Tuning

Simon E Doubt SSCarpal Tunnel Points: 4191 More actions · Answer 6

Just as a follow up, Mark I used your CROSS APPLY syntax and it worked perfectly - allowing me all the groupings I needed.

Thanks again everyone for the expert help.

-Simon

Hari.Sharma SSCertifiable Points: 6904 More actions · Answer 7

Dear Friend,

I think the simplest query for your question is as mentioned below:

-------------------------------------------------------------

Select T2.RetailerID, T2.RetailerIDTheirs

From TableA T1

Inner Join

(Select *,Rank() Over (Partition By RetailerID Order by RetailerIDTheirs) As Rank from TableB) T2

ON T1.RetailerID=T2.RetailerID

Where Rank=1

Cheers,
Hari
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