SQL - Reference Table has Information based on Date - sql-server

Question

So I have a reference table which stores the primary key, description and update date columns. Something like this
SELECT * FROM tblReasonRef
ReasonCode Description UpdateDate
27 Lunch 2010-12-01
24 Meeting 2010-12-01
20 SpecialProj 2010-12-01
The other day, the code description was changed. So now the query returns the following...
ReasonCode Description UpdateDate
27 Lunch 2010-12-01
24 Meeting 2010-12-01
20 SpecialProj 2010-12-01
27 Training 2012-06-22
24 Meeting 2012-06-22
20 Lunch 2012-06-22
The source data tracks every 30 minutes what state a staff member might go into, so you would have the following query...
SELECT * FROM tblhActivity
MemberID Date Time ReasonCode ReasonDuration
10922 2012-06-21 1200 27 100
10922 2012-06-21 1500 24 1800
10922 2012-06-25 1230 27 100
So originally, the query I had was...
SELECT a.MemberID, a.Date, a.Time, r.Description, a.ReasonDuration
FROM tblhActivity a
INNER JOIN tblReasonRef r ON a.ReasonCode = r.ReasonCode
Which worked fine until the change on the 22nd. Now I have two definitions of each code. The question is, how can create a query that will pick the right code depending on the date.
For example, I know that when the date is the 21st, the description for code 27 should be lunch. On the 25th, the description returned should be Training.
Keep in mind also, that this will probably happen again where codes are added to the reference table. I am trying to think the join should also be on UpdateDate but I have to know the start and end date of each reference code. Is there a simple solution?

Answer 1

You really need the start and end dates for the period in which a particular reason is applicable. You can either modify your tblReasonRef to include these dates (best option) or you will need to calculate them.
The following query will calculate an end date for each reason as the day before a new entry for the ReasonCode is added.
SELECT ReasonCode
,Description
,UpdateDate StartDate
,DATEADD(d, -1, UpdateDate) PreviousEntryEndDate
,ROW_NUMBER() OVER(PARTITION BY ReasonCode ORDER BY UpdateDate) AS Row
INTO #reason
FROM tblReasonRef
SELECT a.MemberID
,a.Date
,a.Time
,reason.ReasonCode
,a.ReasonDuration
FROM tblhActivity a
INNER JOIN #reason reason
ON a.ReasonCode = reason.ReasonCode
LEFT JOIN #reason nextReason
ON reason.Row = nextReason.Row - 1
AND reason.ReasonCode = nextReason.ReasonCode
WHERE a.Date BETWEEN reason.StartDate AND ISNULL(nextReason.PreviousEntryEndDate, a.Date)
DROP TABLE #reason

Answer 2

If you modify your table tblReasonRef, like this:
ReasonCode, Description, StarDate, EndDate
you can do this SQL Query:
SELECT a.MemberID, a.Date, a.Time, r.Description, a.ReasonDuration
FROM tblhActivity a
INNER JOIN tblReasonRef r ON a.ReasonCode = r.ReasonCode
WHERE a.Date between r.StartDate and r.EndDate
Remember that you need your code and your model simple.