Trigger and update to a row in SQL Server after it’s been updated

Is this the best way to keep a simple track of changes to a database row:

ALTER TRIGGER [dbo].[trg_121s] 
ON  [dbo].[121s]
  AFTER UPDATE
AS 
BEGIN
-- SET NOCOUNT ON added to prevent extra result sets from
-- interfering with SELECT statements.
SET NOCOUNT ON;

-- Insert statements for trigger here

update dbo.[121s]
set modified=getdate()
where id in 
(select distinct ID from Inserted)

END

So any update to a row in [121s] will result in the modified column being updated.

It works, but I’m not sure if it’s the best way to achieve this.

I’m a litle confused over this line:

(select distinct ID from Inserted)

…and how it knows it’s getting the correct row ID.

Thanks for any confirmation/clarification,

Mark

Answer

inserted is a pseudo-table and it definitely contains all the right rows that were affected by the UPDATE statement (and I assume DISTINCT isn’t necessary, if ID a primary key – though it’s hard to tell what the table is with a name like 121s). Whether all of them actually had changed values is another thing you may consider validating before applying the modified date/time. Barring that, I would probably do it this way:

ALTER TRIGGER [dbo].[trg_121s] 
ON [dbo].[121s]
AFTER UPDATE
AS 
BEGIN
  SET NOCOUNT ON;

  UPDATE t SET modified = CURRENT_TIMESTAMP
   FROM dbo.[121s] AS t
   WHERE EXISTS (SELECT 1 FROM inserted WHERE ID = t.ID);
   -- WHERE EXISTS is same as INNER JOIN inserted AS i ON t.ID = i.ID;
END
GO

If you want to have a 100% foolproof guarantee that they’re all updated with the same timestamp (though I don’t know if I’ve ever seen multiple values in this use case):

ALTER TRIGGER [dbo].[trg_121s] 
ON [dbo].[121s]
AFTER UPDATE
AS 
BEGIN
  SET NOCOUNT ON;

  DECLARE @ts DATETIME;
  SET @ts = CURRENT_TIMESTAMP;

  UPDATE t SET modified = @ts
   FROM dbo.[121s] AS t
  INNER JOIN inserted AS i 
  ON t.ID = i.ID;
END
GO

And if you want to ensure that the update only occurs if, say, the column foo changed value, you could say:

  UPDATE t SET modified = @ts
   FROM dbo.[121s] AS t
   INNER JOIN inserted AS i
   ON t.ID = i.ID
   AND t.foo <> i.foo;

That’s the general pattern, but it becomes more complex if foo is nullable, since SQL Server won’t be able to match on rows where one side has a value and the other doesn’t (or both don’t). In that case you would do this:

   AND 
   (
     t.foo <> i.foo
     OR (t.foo IS NULL AND i.foo IS NOT NULL)
     OR (t.foo IS NOT NULL AND i.foo IS NULL)
   );

Some people will say “I can just use COALESCE or ISNULL against some magic value” like this:

WHERE COALESCE(t.foo, 'magic') <> COALESCE(i.foo, 'magic')

…and I will warn you against this, because you’ll constantly be searching for some magic value that can’t exist in the data.