Question

I have a stored procedure that says

update table1 set value1 = 1 where value1 = 0 and date < getdate() 

I have a trigger that goes like

CREATE TRIGGER NAME ON TABLENAME
FOR UPDATE
...
if UPDATE(value1) 
BEGIN
--Some code to figure out that this trigger has been called
-- the value is always null
END

Any idea why this trigger is called even when the stored procedure does not update any values?

Answer 1

Read the books online article about update(), it’s actually a good read.
You can find it here, http://msdn.microsoft.com/en-us/library/ms187326.aspx.

To answer:
Update() will return true if the column is “updated,” even if no rows were affected by the update.

For example (setup):

If OBJECT_ID('TestTrigger', 'U') is not null drop table TestTrigger
GO
create table TestTrigger(
  ID int identity(1,1)
, Col1 int
, Col2 int
, Trig varchar(1) default 'N' 
)
Insert into TestTrigger(Col1, Col2) values (1,1)
Insert into TestTrigger(Col1, Col2) values (1,2)
GO
IF OBJECT_ID('Testing', 'TR') is not null drop trigger Testing
GO
Create Trigger Testing on TestTrigger
For Update
as
Begin
If Update(Col1)
    begin
        Update TestTrigger Set Trig='Y' where ID=(Select ID from Inserted)
    end
End 

If I run Update TestTrigger Set Col1=0 where Col2=1 and check my rows affected I’ll see

(1 row(s) affected)
(1 row(s) affected)

Which we know is true, 1 is affected by the update statement, the other by the trigger.
If I run Update TestTrigger Set Col1=0 where Col2=10 I’ll see,

(0 row(s) affected)
(0 row(s) affected)

This means that Update(Col1) is returning as true (because otherwise we’d only get a single rows affected returned).

If I run Update TestTrigger Set Col2=0 where Col2=10 I’ll see

(0 row(s) affected)

This is because Col1 is no longer referenced. As @Schar mentioned checking @@RowCount is a good idea as well, or alternatively you could use something like I did (linking inserted).

Kris

EDIT: and to add, from the Create Trigger BO article (http://msdn.microsoft.com/en-us/library/ms189799.aspx),

These triggers fire when any valid
event is fired, regardless of whether
or not any table rows are affected.