TSQL After Update Trigger check for update on multiple columns in one IF UPDATE

Question

The basic syntax for an After Update trigger in TSQL is:

CREATE TRIGGER [dbo].[triggerName]
   ON [dbo].[Table]
   AFTER UPDATE, INSERT --trigger when Update or Insert in table
AS BEGIN
    SET NOCOUNT ON;
    IF UPDATE (ColumnA) -- if ColumnA updates start
    begin
        UPDATE Table
        SET ColumnC = ColumnA + ColumnB -- recalculate Column C
    end 
    IF UPDATE (ColumnB) -- if ColumnB updates start
    begin
        UPDATE Table
        SET ColumnC = ColumnA + ColumnB -- recalculate Column C
    end 
END

Now above could work I guess, but is it possible to combine both IF UPDATES in one?: Something like this:

IF UPDATE (ColumnA) OR (ColumnB) -- if ColumnA or ColumnB updates start
        begin
            UPDATE Table
            SET ColumnC = ColumnA + ColumnB -- recalculate Column C
        end 

You could use a computed column of course, but out of curiosity I'd like to know if you can check for updates on multiple columns at once in a trigger and then do the after update trigger modification.

Answer 1

This is a bit long for a comment.

UPDATE() is a function used in a trigger. There is not specific syntax as if UPDATE(). As far as I know, triggers allow you to combine multiple conditions in an if statement.

But, even more to the point, read the documentation on UPDATE(). The very example used in the documentation is:

IF ( UPDATE (StateProvinceID) OR UPDATE (PostalCode) )
BEGIN
    RAISERROR (50009, 16, 10)
END;

So, use the correct syntax for yours and you will be fine:

IF UPDATE(ColumnA) OR UPDATE(ColumnB) -- if ColumnA or ColumnB updates start
    begin
        UPDATE Table
        SET ColumnC = ColumnA + ColumnB -- recalculate Column C
    end 

However, I would suggest that you use a computed column instead.

Answer 2

You can use COLUMNS_UPDATED:

Returns a varbinary bit pattern that indicates the columns in a table or view that were inserted or updated.

And just check for any of the bits you're interested in being set using bitwise operators:

IF (COLUMNS_UPDATED() & CAST(0x0A as int)) != 0
begin
   --column 2 or 4 was updated
end

Of course, this does then depend on the column order, which is usually a bad thing to depend upon, and it also won't blow up if the column order is later changed.