Question

order by in subquery

Asked by: mock5c

Is there any reason why or why not you should do an 'order by' in a subquery?  Performance increase if you do?  Output is the same either way, right?

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Asked On
2006-12-28 at 10:16:43ID22105291
Tags

order

,

subquery

Topic

Databases Miscellaneous

Participating Experts
4
Points
50
Comments
8

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Answers

 

by: Arthur_WoodPosted on 2006-12-28 at 10:20:39ID: 18209488

depends on how you are using the subquery, but in general, an order by clause will adversely affect performance.

AW

 

by: tonyteriPosted on 2006-12-28 at 10:22:14ID: 18209497

You can do, but as stated above performance will be degradated

/TT

 

by: acperkinsPosted on 2006-12-28 at 10:36:28ID: 18209572

Not sure what DBMS you are using, but in T-SQL and ORDER BY is only supported with a TOP clause.

 

by: Arthur_WoodPosted on 2006-12-28 at 12:10:12ID: 18210164

acperkins>> where did that come from.  You can use an Order By clause in *any* SQL statement

here is a line for a SQL Server DB that I recently created:

select * from FAQ order by author desc;

 (runs just fine in Query Analyzer)

here is the help file from Books on Line (Transact-SQL) for the Order BY clause:

ORDER BY
Specifies the sort order used on columns returned in a SELECT statement. For more information, see SELECT


AW

 

by: awking00Posted on 2006-12-28 at 12:38:51ID: 18210365

Because order by can degrade performance, the general rule is to use it when it is necessary and don't use it if it has no meaning. Note, there are some sql statements such as union where order by is not allowed.

 

by: acperkinsPosted on 2006-12-28 at 15:04:38ID: 18211210

>>acperkins>> where did that come from.<<
The original question stated (my enfasis):
"Is there any reason why or why not you should do an 'order by' ***in a subquery***?"

Now, try and post a valid query with an ORDER BY with a subquery and no TOP clause.

 

by: acperkinsPosted on 2006-12-28 at 15:07:02ID: 18211221

awking00,

>>Note, there are some sql statements such as union where order by is not allowed.<<
This is not strictly correct.  There is nothing wrong with:

SELECT A, B, C
FROM TableA
UNION ALL
SELECT X, Y, Z
FROM TableB
ORDER BY 1

But I believe I understand what you meant.

 

by: acperkinsPosted on 2006-12-29 at 18:56:06ID: 18217611

>>Now, try and post a valid query with an ORDER BY with a subquery and no TOP clause.<<
Yeah, I did not think so, either  :)

20120131-EE-VQP-002

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