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Stupid CTE tricks -- string concatenation

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25 Dec 2011CPOL 39.6K   9   3
Concatenating values from multiple rows into one string value via a Common Table Expression
If you have a table like:

Make    Model
------- -----------
Toyota  Yaris
Toyota  Corrolla
Ford    Pinto
Ford    Model A
Ford    Thunderbird
Nash    Rambler


and you want something like:

Make    Models                          Count
------- ------------------------------- -----
Ford    Thunderbird , Pinto , Model A   3
Nash    Rambler                         1
Toyota  Yaris , Corrolla                2


you'll need to write some code -- SQL Server doesn't have that built in (yet).
You can write a function to do it, you can write a custom CLR aggregator to do it, but I wanted to see if I could use a Common Table Expression to do it.

The following is such a CTE:

WITH cte AS
(
  SELECT Make
  , COUNT(*) [Count]
  , MAX(Model) Models
  , 0 [Rank]
  FROM MakeModel
  GROUP BY Make
UNION ALL
  SELECT cte.Make
  , cte.[Count]
  , Models + N' , ' + Model
  , [Rank] + 1
  FROM cte
  INNER JOIN MakeModel
  ON cte.Make=MakeModel.Make
  AND cte.Models NOT LIKE '%' + Model + '%'
  AND cte.[Rank]<cte.[Count]
)
 
SELECT Make,MAX(Models) Models,MAX([Count]) [Count] FROM cte GROUP BY Make ORDER BY Make


Caveats:
0) You may need to CONVERT your values to NVARCHAR(MAX)
1) You may need to protect against NULL values
2) I don't think this will scale well
3) I can't think of a way to get the values listed ascending rather than descending

This was mainly an interesting exercise, I don't expect to use it in production systems.

License

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)


Written By
Software Developer (Senior)
United States United States
BSCS 1992 Wentworth Institute of Technology

Originally from the Boston (MA) area. Lived in SoCal for a while. Now in the Phoenix (AZ) area.

OpenVMS enthusiast, ISO 8601 evangelist, photographer, opinionated SOB, acknowledged pedant and contrarian

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Comments and Discussions

 
GeneralFor ordering ascending, try changing AND cte.[Rank] < ct... Pin
jsc423-Jan-12 1:11
professionaljsc423-Jan-12 1:11 
General0) your input tables do have to be converted to nvarchar(max... Pin
Daniel Gidman21-Dec-11 7:57
professionalDaniel Gidman21-Dec-11 7:57 
GeneralRe: 0) I know the ones shown don't 1) I think they might, but I ... Pin
PIEBALDconsult21-Dec-11 16:48
mvePIEBALDconsult21-Dec-11 16:48 

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