sql join

Table A has following fields and data
CID, Addkey, Cplan, changeDt
1,1a, planA, 10/10/2010
2, 1b, planD, 12/10/2010
3, 2a planE, 07/15/2010
4, 1a, planA, 06/22/2010
5, 1b, planD, 10/23/2010
6 1c, planA, 12/10/2010
7, 1a, planA, 06/06/2011

I want to find all CIDs where Addkey, Cplan are same and set them as master and child CID.
MasterCID, ChildCID. The CID with most recent changeDt should be the masterCID and the other one will be the childCID. ex

MasterCID, ChildCID
1,4
1,7
5,2

How do I do that using sql (MS Sql Server 2008)?

Thanks.

patd1Asked:
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jogosConnect With a Mentor Commented:
First identify the most recent date for each combination (subquery) and then find the matching records
select x.cid,y.cid
from (select cid,addkey,cplan, max(changedt)
from a group by addkey,cplan
) a as x 
inner join a as y on ( x.addkey = y.addkey 
                          and x.cplan = y.cplan
                          and x.changedt > y.changedt)

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deightonConnect With a Mentor progCommented:
SELECT SUBQ.CID AS MASTERCID, T1.CID FROM TABLEA T1 JOIN
(select *, row_number() OVER(PARTITION BY ADDKEY,CPLAN ORDER BY ChangeDT,cid) AS rown FROM tablea) SUBQ
ON SUBQ.rown=1 AND SUBQ.ADDKEY = T1.ADDKEY AND SUBQ.CPLAN = T1.CPLAN 
	AND SUBQ.CID <> T1.CID

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deightonConnect With a Mentor progCommented:
or perhaps

SELECT SUBQ.CID AS MASTERCID, T1.CID FROM TABLEA T1 JOIN
(select *, row_number() OVER(PARTITION BY ADDKEY,CPLAN ORDER BY ChangeDT DESC,cid) AS rown FROM tablea) SUBQ
ON SUBQ.rown=1 AND SUBQ.ADDKEY = T1.ADDKEY AND SUBQ.CPLAN = T1.CPLAN
      AND SUBQ.CID <> T1.CID
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