Question

I have the following tables:

1) TBL (ID, Data, LastUpdated, DateCreated)
2) TBL_LOG(Log_ID, LogCreateDate, ID, Data, LastUpdated, DateCreated)

TBL used to store current data and TBL_LOG used to store every change of data in TBL (using update trigger, the deleted record from TBL copied to TBL_LOG).

Now I need to retrieve the data by relevance date.
How I can write the table valued function (or other alternative) that will return the data depending on RelevanceDate.

I understand the logic, but I can’t find a good way to write it in SQL…
The logic looks like:

@RelevanceDate = '2011-03-01'
IF TBL.LastUpdated <= @RelevanceDate THEN return data from TBL
ELSE IF Exists data in TBL_LOG where TBL_LOG.LastUpdated <= @RelevanceDate 
    THEN return most resent data from TBL_LOG where TBL_LOG.LastUpdated <= @RelevanceDate
ELSE IF Exists data in TBL_LOG where TBL_LOG.LastUpdated > @RelevanceDate 
    THEN return the oldest data from TBL_LOG.LastUpdated > @RelevanceDate
ELSE return data from TBL

This function should return data using the logic above, for all records in TBL.

In short words, for each row in TBL i need data that was on “RelevanceDate”.

Answer 1

At the end of my investigation, i ended up with something like folowing:

DECLARE @RelevanceDate DATETIME
SET @RelevanceDate = '2012-03-01'

SELECT 
        MainData.ID, MainData.Data, MainData.LastUpdated, MainData.DateCreated
FROM   (
           SELECT 
                  *
                  ,ROW_NUMBER() OVER(PARTITION BY AllData.ID, AllData.RowNum ORDER BY AllData.LastUpdated) AS MainRowNumber
           FROM   (
                      --Current Data
                      SELECT 
                             NULL LogID
                            ,NULL LogCreateDate
                            ,*
                            ,ROW_NUMBER() OVER(PARTITION BY ID ORDER BY LastUpdated DESC) AS RowNum
                      FROM   TBL
                      WHERE  (lastupdate<=@RelevanceDate)

                      UNION

                      --History Data
                      SELECT  
                            *
                            ,ROW_NUMBER() OVER(PARTITION BY ID ORDER BY LastUpdated) AS RowNum
                      FROM   TBL_LOG
                      WHERE  lastupdate>@RelevanceDate
                  ) AllData WHERE AllData.RowNum=1
       ) MainData
WHERE  MainData.MainRowNumber = 1

If someone have a suggestion how it can be done in better way, i will glad to see it 🙂