Multiple SQL UPDATE statements OR Single with CASE in it?

Question

I've built a trigger where I am trying to keep priorities in order and not allow duplicate priority values. There are some things to consider.

1. The user can free form the priority in.
2. There is nothing blocking them from picking the same priority as another item.

3. When a value is entered equal to another value the newly prioritized item should take precedence in the priority and the other should be incremented.

   CREATE TRIGGER dbo.trg_Priority
       ON  dbo.Stories
       AFTER INSERT,UPDATE,DELETE
   AS
   BEGIN
       SET NOCOUNT ON;
   
   -- Insert statements for trigger here
   DECLARE @StoryId INT
   DECLARE @OldLocation INT
   DECLARE @NewLocation INT
   
   SELECT @NewLocation = Priority, @StoryId = StoryId FROM INSERTED
   SELECT @OldLocation = Priority FROM DELETED
   
   IF @NewLocation = @OldLocation
       RETURN;
   
   IF @NewLocation IS NULL
   BEGIN
       UPDATE Stories SET
           Priority = Priority - 1
       WHERE Priority > @OldLocation
   END
   
   IF @NewLocation > @OldLocation
   BEGIN
       UPDATE Stories SET
           Priority = Priority + 1
       WHERE Priority >= @NewLocation
       AND StoryId <> @StoryId
   END
   
   IF @NewLocation < @OldLocation
   BEGIN
       UPDATE Stories SET
           Priority = Priority + 1
       WHERE Priority >= @NewLocation
       AND Priority < @OldLocation
       AND StoryId <> @StoryId
   END
   END
   GO
   

I haven't tested this trigger a whole lot so if there's areas of concern feel free to speak up. What I ultimately want to know is if I should try and convert this to a single update with a case statement. (If that's even possible.)

If it would be more performant to make this a single UPDATE statement I could really use a hand figuring it out!

Answer 1

You need to rewrite the trigger. It assumes only one records will ever be inserted/updated or delted at atime. You cannot write a trigger with that assumption, triggers operate on batches of data not row-byrow. You need to join to inserted and deleted in your updates. So yes, I would try to write the update with a case statement.

And why is this a deleted trigger? There won;t be arecord to update if it was deleted.

Answer 2

Make sure you rollback and raise an error (severity 16) if @@ROWCOUNT > 1. Your trigger as currently written would cause a great deal of data corruption of the user attempted to insert, update or delete multiple rows at once.

That said, IronGoofy is right: you will only be touching the table once, regardless if the condition. So, breaking it out into multiple statements makes the code easier to read/more maintainable.

If you were to allow multiple rows to be updated at once, you would need to change this. The logic may be daunting!

Answer 3

How about this:

 UPDATE Stories SET Priority = CASE 
     WHEN Priority > @OldLocation THEN Priority-1
     WHEN Priority >= @NewLocation AND StoryID <> @StoryID THEN Priority+1
     WHEN Priority >= @NewLocation AND @Priority < @OldLocation And StoryID <> StoryID THEN Priority +1
END
GO

Answer 4

There shouldn't be any (noticeable) performance difference, in any case you will be issuing a single update statement to the database.

Answer 5

I decided to ditch this idea, and I leave it up to the UI to update the priority. I've stopped allowing user input.

I'm not sure which of these answers are correct, so I'm going to mark this as correct and let the community figure it out with trial and error. :)