Question

I have an MS Access 2007 database that I need to create an update for. The table I am trying to update looks like this:

 CarID  WeekOf          NumDataPoints   NumWksZeroPoints
 3AA    May-14-2011     23              0
 7BB    May-14-2011     9               0

 3AA    May-21-2011     35              0
 7BB    May-21-2011     0               1

 3AA    May-28-2011     24  
 7BB    May-28-2011     0   

I am processing the latest recordset of May-28-2011 and the gist is to update each car with the number of weeks its had no data points. I do this by checking the current week number of points and if it does have some points then the #WeeksZeroPoints gets set to zero, and if the current number of points is zero then i take the prior weeks count and increment by one. For my last week I would have input

  0
  2

So I have tried something like

UPDATE tblCars 
SET NumWksZeroPoints = IIF(NumDataPoints<>0, 0, (SELECT MAX(NumWksZeroPoints) AS wzp 
                                                 FROM tblCars AS f 
                                                 WHERE f.CarID=tblCars.CarID AND 
                                                       f.WeekEnding=#5/21/2011#) + 1
                       )
WHERE WeekOf=#5/28/2011#;

Unfortunately this doesn’t work like I thought it would. I think I have the concept down and most of the SQL, I just cant seem to make it work. This is against MS Access so some of the other tricks I know just don’t work. Any help appreciated.

Answer 1

Using your sample data I ran the following

UPDATE tblcar AS c 
       INNER JOIN tblcar AS previous 
         ON c.carid = previous.carid 
SET    c.numwkszeropoints = Iif([previous].[NumWksZeroPoints] = 0, 0, 
                                   [previous].[NumWksZeroPoints] + 1) 
WHERE c.weekof =#5/28/2011 # 
         AND previous.weekof =#5/21/2011#;

The table afterwards looked like this

CarID WeekOf     NumDataPoints NumWksZeroPoints
----- ---------- ------------- -----------------
3AA   05/14/2011 23            0
7BB   05/14/2011 9             0
3AA   05/21/2011 35            0
7BB   05/21/2011 0             1
3AA   05/28/2011 24            0
7BB   05/28/2011 0             2

Basically the query does a self join back to the previous week, and the update the current week to the previous week’s value + 1 if its not zero.