Fun With SQL Server Update Triggers

Below is some code that will show how to test for updated field values in an update trigger. As you can see the IF UPDATE (field) is true even when the values don’t change. Another thing to keep in mind is that if a value changes from NULL to something else and vice-versa, and you are comparing deleted and inserted tables without using COALESCE or ISNULL it won’t return those rows. Run the code below to see what I mean


CREATE TABLE TestTrigger (TestID INT identity,
name VARCHAR(20),
value DECIMAL(12,2) ,
CONSTRAINT chkPositiveValue CHECK (value > 0.00) )


INSERT INTO TestTrigger
SELECT 'SQL',500.23


CREATE TRIGGER trTest
ON TestTrigger
FOR UPDATE
AS

IF @@ROWCOUNT =0
RETURN

IF UPDATE(value)
BEGIN
SELECT '1', * FROM deleted d JOIN inserted i ON d.testid =i.testid
SELECT '2',* FROM deleted d JOIN inserted i ON d.testid =i.testid
AND i.value <> d.value
SELECT '3',* FROM deleted d JOIN inserted i ON d.testid =i.testid
AND COALESCE(i.value,-1) <> COALESCE(d.value,-1)
END
GO

--Let's update the value to 100
UPDATE TestTrigger SET value = 100 WHERE testid =1
--we get back all 3 rows


--Let's run the same statement
UPDATE TestTrigger SET value = 100 WHERE testid =1
--we get back the first row


--Let's really update
UPDATE TestTrigger SET value = 200 WHERE testid =1
--we get back all 3 rows

--Let's update with NULL
UPDATE TestTrigger SET value =NULL WHERE testid =1
--we get back rows 1 and 3, row 2 is not returned because it can't compare it

--Let's update with NULL again
UPDATE TestTrigger SET value =NULL WHERE testid =1
--we get back row 1

--Let's update with 300
UPDATE TestTrigger SET value =300 WHERE testid =1
--we get back rows 1 and 3, row 2 doesn't return because it can't compare NULL to 300

--Let's update with 500
UPDATE TestTrigger SET value =500 WHERE testid =1
--we get back all 3 rows