Trigger To Prevent Duplicates

Hi all,

I'm writing a trigger to prevent duplicates. I know that this can be done through primary key or unique constraints but in the real world my uniqueness is defined by 8 columns which is too a big an index to maintain on the primary / unique key.

If I create a table with 2 columns

CREATE TABLE Table1
(CentreCHAR(10),
Month CHAR(3)
)


Then create a trigger to prevent duplicates

CREATE TRIGGER trigger_Check_Duplicates
ON Table1
FOR INSERT, UPDATE
AS
-- This trigger has been created to check that duplicate rows are not inserted into AudioVisual table.
DECLARE @IsDuplicate INTEGER
-- Check if row exists
SELECT @IsDuplicate = 1
FROM Inserted i, Table1 t
WHERE t.Centre = i.Centre
AND t.Month = i.Month
IF (@IsDuplicate = 1)
-- Display Error and then Rollback transaction
BEGIN
RAISERROR ('This row already exists in the table', 16, 1)
ROLLBACK TRANSACTION
END

Then insert a row into the new table (no other data is in there)

INSERT Table1
VALUES('0691040176','AUG')

I get the Trigger error message that the row already exists. Why is this the case? I though that Table 1 (target table) would show no entries as it has no data - it should be a before image of the table and the inserted table should be an after image.


Please help!!!


Thanks

Neill