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