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Stupid CTE tricks -- string concatenation

, 25 Dec 2011
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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.

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About the Author

PIEBALDconsult
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
 
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Comments and Discussions

 
GeneralFor ordering ascending, try changing AND cte.[Rank] < ct... Pinmemberjsc423-Jan-12 1:11 
General0) your input tables do have to be converted to nvarchar(max... PinmemberDaniel Gidman21-Dec-11 7:57 
GeneralRe: 0) I know the ones shown don't 1) I think they might, but I ... PinmemberPIEBALDconsult21-Dec-11 16:48 

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