Question

Using COALESCE to concatenate strings -- what about sequence?

Asked by: DanielWilson

I just saw COALESCE used to concatenate strings.

http:/Q_23242947.html

I have a situation where there's a sequence applied, thus:

OrderID nvarchar(10)
Seq tinyInt
commentText nvarchar(25)

With sample data like:
OrderID        Seq         commentText
Or1234          1             Ship it as soon as possible
OR1234          2            because we need it yesterday
OR987           1             Ship Complete.
OR987            2             No backorders.
OR987            3            Use UPS Next Day.

So the question:  Can a COALESCE solution like AngelIII's (below) from the other question be made to respect the Sequence?

Thanks!

CREATE FUNCTION dbo.GetData(@id int) 
RETURNS VARCHAR(1000)
AS
BEGIN
  DECLARE @res VARCHAR(1000)
 
  SELECT @res = COALESCE(@res + ', ' , '') + fibretype + ' ' + cast(FibrePercent as varchar(10)) + '%' as PercentDisplay
    FROM yourtable
  WHERE id = @id
 
  RETURN @res 
END

                                  
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Asked On
2008-03-14 at 15:55:11ID23243291
Topics

SQL Server 2005

,

MS SQL Server

,

SQL Server 2008

Participating Experts
3
Points
250
Comments
10

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Answers

 

by: BriCrowePosted on 2008-03-14 at 16:06:01ID: 21130403

Just add an ORDER BY clause

SELECT @res = COALESCE(@res + ', ' , '') + fibretype + ' ' + cast(FibrePercent as varchar(10)) + '%' as PercentDisplay
    FROM yourtable
  WHERE id = @id
ORDER BY Seq

 

by: dportasPosted on 2008-03-14 at 16:07:46ID: 21130408

That function doesn't use COALESCE to concatenate a string. It concatenates using a variable assignment in the SELECT. Unfortunately the result of assignment in a multi-row SELECT is officially undefined. You could try adding ORDER BY to that statement but there are no guarantees and even if it works today it could break in future under some service pack or hotfix or even for no apparent reason at all. I definitely don't recommend using this kind of kludge.

Instead try:

SELECT DISTINCT OrderId,
 STUFF(
 (SELECT ' '+CommentText AS [text()]
 FROM Orders
 WHERE OrderId = T.OrderId
 ORDER BY seq
 FOR XML PATH( '' )
 ), 1,1,'') AS concat
 FROM Orders AS T;

 

by: RacimoPosted on 2008-03-15 at 09:35:35ID: 21133268

<<Can a COALESCE solution like AngelIII's (below) from the other question be made to respect the Sequence?>>
No. ORDER BY does.

 

by: DanielWilsonPosted on 2008-03-15 at 15:53:25ID: 21134670

Yes, Racimo, obviously it's ORDER BY that causes records to be ORDERed BY a particular field.

I guess I should have asked whether a solution like AngelIII's can be written, using COALESCE to combine the records and using ORDER BY to sequence the results.  ORDER BY, as we all know, cannot be used effectively in all situations.

And ... I think you're all saying that it will work under SQL Server ... currently.  But there's no guarantee it will in the future.

I'm going to try out the solution using STUFF and FOR XML ... though I'll admit I never saw FOR XML used in that way!

Anyway, thanks to all for the responses.  I'll test some of the ideas & glad distribute points on Monday/Tuesday.

 

by: RacimoPosted on 2008-03-15 at 16:09:23ID: 21134710

<<I'm going to try out the solution using STUFF and FOR XML ... though I'll admit I never saw FOR XML used in that way!>>
Quite frankly, I do not see what XML can bring to your current problem appart from additional unessecary complexity.

 

by: dportasPosted on 2008-03-16 at 02:26:42ID: 21136263

Daniel, Disregard my previous answer and try the following query. Better still, replace T1 with a table in which OrderId is unique (assuming you have such a table).

Racimo, The main advantage of the FOR XML solution is that it is documented and supported. Concatenation using variables in a SELECT statement on the other hand is an undocumented kludge the results of which depend on the execution plan chosen by the optimiser. Its behaviour is therefore undefined and unpredictable. There are circumstances where it will break and not return a concatenated result at all.

SELECT OrderId, STUFF(Concat,1,1,'')
 FROM
(SELECT DISTINCT OrderId
 FROM Orders) T1
 CROSS APPLY
(SELECT ' '+CommentText AS [text()]
 FROM Orders
 WHERE OrderId = T1.OrderId
 ORDER BY Seq
 FOR XML PATH( '' )
 ) AS T2(Concat);

 

by: RacimoPosted on 2008-03-16 at 06:34:52ID: 21136859

<<Racimo, The main advantage of the FOR XML solution is that it is documented and supported.>>
OK.  Check the article below..

On the myth of self describing XML
http://www.oceaninformatics.biz/publications/e2.pdf

 

by: dportasPosted on 2008-03-16 at 06:53:47ID: 21136932

Thanks for the link. That's a good little article but it doesn't have anything to do with the solution I posted. My query doesn't even return any XML document. It just uses the FOR XML feature to format a string. Did you try it? Do you have an alternative solution to offer?

 

by: RacimoPosted on 2008-03-16 at 09:18:12ID: 21137325

<<My query doesn't even return any XML document. It just uses the FOR XML feature to format a string.?>>
XML related solutions is simply stacking hacks on the top of kludges.  

The real issue here is why the ORDER BY solution would not work in the future.   You claim that the ORDER BY might not work in future releases for some reasons but I tend to believe that FOR XML type have more chances of being revamped than ORDER BY which have been around for more than 30 years.  Between the two I stick to ORDER BY.  

*XML is just a solution looking for a problem to solve*.

<<Did you try it?>>
No.  Why try a solution in which I do not believe.

<<Do you have an alternative solution to offer?>>
BriCrowe solution is the way to go.  So I do not need to repeat it.   Obscure reasons for not using it and favoring FOR XML just are neither convincing nor preferable.

IMHO.

Regards

 

by: DanielWilsonPosted on 2008-03-20 at 09:57:16ID: 31439931

Thanks!  The Coalesce with Order By is working for me.  I'm going to revisit the XML solution and test & understand it a little later.

20120131-EE-VQP-002

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