Question

I have a TSQL SELECT that can return a null. I tried using ISNULL to replace it with 0 but for some reason it is not working. The table selecting from has the following columns:

• storeID –> int
• penaltyPercent –> decimal(9,2)
• penaltyDate –> dateTime

SELECT  ISNULL(penaltyPercent, 0.0) AS penaltyPercent   FROM    dbo.Penalty   WHERE   (penaltyDate = (SELECT     MAX(penaltyDate) AS date                           FROM       dbo.Penalty AS Penalty_1                           WHERE      (penaltyDate <= @date) AND (storeID = @storeID))) AND         (storeID = @storeID)  

When the date is before the first penalty date (when there should be 0 penalty), no result is returned. Not sure why this doesn’t work. I have a work around but it is bugging me.

Here is a sample of the data being used:

storeID  penaltyDate             penaltyPercent   182      10/1/2008 12:00:00 AM   0.020000   182      11/1/2008 12:00:00 AM   0.040000   182      12/1/2008 12:00:00 AM   0.070000   

Answer 1

When you say ‘When the date is before the first penalty date’, do you mean when the value of @date is less than the value returned from this query?

SELECT MIN(penaltyDate) FROM Penalty 

Because then your inner query is going to return null, and (if you’re using ANSI nulls) this part will return false,

WHERE (penaltyDate = ... 

Because comparisons with null always return null. So instead of selecting one row with a null value, you’re selecting no rows.

Addendum:

To confirm this is the problem, change your outer query’s first line to

SELECT ISNULL(MAX(penaltyPercent),0.0) AS penaltyPercent 

It will work because of what I’ve described above.