Thursday, March 11, 2010

Concatenating row values in a single statement using CROSS APPLY

One more SQL issue, one more solution provided. I feel good about it now ;). I am adding it to my collection because it really helps in reducing the duration to execute the process. It is about a new feature which was introduced way back in SQL 2005. CROSS APPLY.

The Scenario: Let me lay down the scenario, I have a table which has some 100 thousand + records. This is a very (deliberately) de normalized table. The structure and contents of the table are as below.




IDFirstNameEducation
1Vighnesh
BSC
KUD
1995


2Vighnesh


MSC
KUD
1997

3Vighnesh

PHD
MIT
2009

4Mark


BCOM
KUD
2000

5Mark
 
MBA
IIMB
2008

6Vicky


BE
VIT
2008




From the above table, I want to get only distinct records, with the education column concatenated. Something like this…



ID

FirstName

Education

1

Vighnesh

 BSCKUD
1995
MSC
KUD1997
PHDMIT2009

4

Mark


BCOMKUD
2000
MBAIIMB2008


6

Vicky


BE
VIT2008




This can be achieved with a simple query…


SELECT DISTINCT FirstName, CAST(Concat_Education as nvarchar(max)) as Education_ctn
FROM EducationHistory p1
CROSS APPLY
(
SELECT cast(Education + '' as xml)
FROM EducationHistory p2
where p2.Name = p1.Name
ORDER BY Education
FOR XML PATH(''), TYPE
)
V ( Concat_Education )


In the above query we are using FOR XML PATH(‘’), TYPE. If we don’t use this along with casting the Education column as xml (cast(Education + '' as xml)), we will get the output xml as “<” and “>”.
While searching for the possible solution, I searched the web with different approaches. This also shows the evolution in my thinking from start to end. Sample this:
1. assigning value to variable in sub query
2. can I use variable in sub query?
3. coalesce in sql server
4. combine value of 2 rows sql
5. combine value of 2 rows sql in DISTINCT query
6. concat subquery rows
7. concat using coalesce in subquery – here is where I got the answer (thanks to this blog)
8. FOR XML PATH('') < >
9. FOR XML PATH('') < with < get rid
10. FOR XML PATH('') is replacing < with <
11. FOR XML PATH('') sql server

Feel free to contact me in case you need help.
-Vighnesh Bendre

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