Query last event each person attended - sql

Question

I have a table called CampRegistration with the following structure:
[EID] INT, [CampName] VARCHAR(100), [StartDt] DATETIME
EID is the identifier of the person who attended the camp. I wish to select the last camp each person in the CampRegistration table has attended prior to a given date.
Here was my first attempt:
#DECLARE #currentCampDt DATETIME = '2012-8-4';
SELECT [EID], [CampName], MAX([StartDt]) [StartDt]
FROM [CampRegistration]
WHERE [StartDt] < #currentCampDt
GROUP BY [EID],[CampName]
order by [EID]
The problem here is that if someone has attended multiple camps w/ different names I'll get multiple results for that person (the last attended of each camp name). I only wish to get a single record back for each person, the last camp of any name they attended. I do ultimately need to get those three pieces of info for each record (EID, CampName, and the camp's StartDt) so I'm not sure if I really can remove CampName from the Group By.
I'm using SQL Server 2012 and would greatly appreciate any suggestions on how to implement this type of query result.
Thanks.

Answer 1

One approach would be to use a CTE (Common Table Expression).
With this CTE, you can partition your data by some criteria - i.e. your EID - and have SQL Server number all your rows starting at 1 for each of those "partitions", ordered by some criteria.
So try something like this:
;WITH CampEvents AS
(
SELECT
EID, CampName, StartDt,
RowNum = ROW_NUMBER() OVER(PARTITION BY EID ORDER BY StartDt DESC)
FROM
dbo.CampRegistration
)
SELECT
EID, CampName, StartDt
FROM
CampEvents
WHERE
RowNum = 1
Here, I am selecting only the "first" entry for each "partition" (i.e. for each EID) - ordered by the descending StartDt - so the newest, most recent event has RowNum = 1.
Does that approach what you're looking for??

Answer 2

This won't be fast, but will do it:
#DECLARE #currentCampDt DATETIME = '2012-8-4';
SELECT o.[EID], o.[CampName], o.[StartDt]
FROM [CampRegistration] o
WHERE o.[StartDt] = (
SELECT MAX(i.[StartDt])
FROM [CampRegistration] i WHERE i.[StartDt] < #currentCampDt
AND
i.[EID] = o.[EID]
)
order by o.[EID]

Answer 3

You can do this:
WITH LastCampsAttended
As
(
SELECT *,
ROW_NUMBER() OVER(PARTITION BY EID ORDER BY StartDt DESC) AS rownum
FROM CampRegistration
WHERE [StartDt] < #currentCampDt
)
SELECT
EID,
CampName,
StartDt
FROM LastCampsAttended
WHERE rownum = 1;
SQL Fiddle Demo
Or:
SELECT
camps.*
FROM CampRegistration camps
INNER JOIN
(
SELECT EID, MAX(StartDt) LatestDate
FROM CampRegistration
GROUP BY EID
) LatestCamps ON camps.EID = LatestCamps.EID
AND camps.StartDt = LatestCamps.LatestDate
WHERE camps.StartDt < #currentCampDt ;
Updated SQL fiddle Demo