Comparing fields in a trigger

Question

So I've created a trigger that compares update before and after and determines if specific fields, specified in the where clause, have changed. If so, I insert a snapshot of the prior information into a history table.

The problem is when the field is created with null values (given the business rules, it is legitimately null) and when an update is made, it is unable to evaluate that the string is not equal to the null value. I want to capture that it was sent to us empty and later filled in.

What approach should I use to compare to null fields without impacting performance?

This is for SQL Server 2005

CREATE TRIGGER [prj].[TRG_Master_Projection_Upd_History] ON [prj].[Master_Projections]
AFTER UPDATE
AS
    SET NOCOUNT ON -- Prevents the error that gets thrown after inserting multiple records 
                   -- if multiple records are being updated

BEGIN


INSERT INTO prj.History_Projections (ProjectionID, Cancelled, Appeal, Description, Response_PatternID, 
                                     Proj_Mail_date, [3602_Mail_Date], Proj_Qty, [3602_Qty], Proj_Resp_Rate,
                                     Bounce_Back, Nickels, Kits, Oversized_RE, ChangeComments, 
                                     Modification_Process, Modification_Date, Modification_User)
    SELECT D.ProjectionID, D.Cancelled, D.Appeal, D.Description, D.Response_PatternID, D.Proj_Mail_Date,
            D.[3602_Mail_Date], D.Proj_Qty, D.[3602_Qty], D.Proj_Resp_Rate, D.Bounce_Back, D.Nickels, D.Kits,
            D.Oversized_RE, D.ChangeComments, D.Modification_Process, D.Modification_Date, D.Modification_User
       FROM deleted as D
    JOIN inserted as I
         ON D.ProjectionID = I.ProjectionID
      WHERE (I.Cancelled <> D.Cancelled
          OR I.Appeal <> D.Appeal
          OR I.Description <> D.Description
          OR I.Response_PatternID <> D.Response_PatternID
          OR I.Proj_Mail_Date <> D.Proj_Mail_Date
          OR I.[3602_Mail_Date] <> D.[3602_Mail_Date]
          OR I.Proj_Qty <> D.Proj_Qty
          OR I.[3602_Qty] <> D.[3602_Qty]
          OR I.Proj_Resp_Rate <> D.Proj_Resp_Rate
          OR I.Bounce_Back <> D.Bounce_Back
          OR I.Nickels <> D.Nickels
          OR I.Kits <> D.Kits
          OR I.Oversized_RE <> D.Oversized_RE )
END;
SET NOCOUNT OFF;

Answer 1

Unfortunately, you really have to use sentinel values

ISNULL(I.Response_PatternID, 0) <> ISNULL(D.Response_PatternID, 0)

There is no magic unfortunately: you have to compare every value to see any differences.

Saying that, you only touch the INSERTED and DELETED tables so as bad as this is, the main table is not touched. Unless you have update that affect 10000s if rows, it will run OK.

You can use OR too to but this is cumbersome

(I.Response_PatternID <> D.Response_PatternID OR I.Response_PatternID IS NULL AND I.Response_PatternID IS NOT NULL OR I.Response_PatternID IS NOT NULL AND I.Response_PatternID IS NULL) 

I'd stick with ISNULL to avoid subtle datatype issues with COALESCE