sql trigger to stop duplicates across row - sql

Question

I have a table with multiple records:
User_Name ( e.g. 'TOM')
Question_ID (eg 'q002')
Answer (e.g. 'D')
i want to create a trigger so that no one can submit an answer to the same question twice.
It has to be a trigger only.
CREATE TRIGGER trigger_Check_Duplicates
ON submit_Answer
FOR INSERT
AS
IF SELECT???
PRINT 'duplicate'
raiserror('cant submit answer to same question twice')
ROLLBACK
End

Answer 1

Create trigger
CREATE TRIGGER dbo.uniqueUserQuestion
ON dbo.submit_Answer
INSTEAD OF INSERT
AS
BEGIN
SET NOCOUNT ON
IF EXISTS
(
SELECT 1
FROM dbo.submit_Answer T
INNER JOIN INSERTED I
ON T.user_name = I.user_name
AND T.question_id = I.question_id
)
BEGIN
-- Do dupe handling here
PRINT 'duplicate'
raiserror('cant submit answer to same question twice')
return
END
-- actually add it in
INSERT INTO
dbo.submit_Answer
SELECT
*
FROM
INSERTED I
END
GO

Answer 2

MySql does not support INSTEAD OF triggers, which is what you'd need to use here. In SQL Server, you'd use an INSTEAD OF INSERT trigger that will fire before the insert occurs, where you can write a check for the duplicate. However, if you can avoid a trigger, why not use a Stored Routine and just check for the duplicate before inserting?
This is, of course, if you really, really cannot use a constraint.
Edit: Updating answer for MSSQL.
Here's an example right from MSDN:
CREATE TRIGGER IO_Trig_INS_Employee ON Employee
INSTEAD OF INSERT
AS
BEGIN
SET NOCOUNT ON
-- Check for duplicate Person. If there is no duplicate, do an insert.
IF (NOT EXISTS (SELECT P.SSN
FROM Person P, inserted I
WHERE P.SSN = I.SSN))
INSERT INTO Person
SELECT SSN,Name,Address,Birthdate
FROM inserted
ELSE
-- Log an attempt to insert duplicate Person row in PersonDuplicates table.
INSERT INTO PersonDuplicates
SELECT SSN,Name,Address,Birthdate,SUSER_SNAME(),GETDATE()
FROM inserted
-- Check for duplicate Employee. If no there is duplicate, do an INSERT.
IF (NOT EXISTS (SELECT E.SSN
FROM EmployeeTable E, inserted
WHERE E.SSN = inserted.SSN))
INSERT INTO EmployeeTable
SELECT EmployeeID,SSN, Department, Salary
FROM inserted
ELSE
--If there is a duplicate, change to UPDATE so that there will not
--be a duplicate key violation error.
UPDATE EmployeeTable
SET EmployeeID = I.EmployeeID,
Department = I.Department,
Salary = I.Salary
FROM EmployeeTable E, inserted I
WHERE E.SSN = I.SSN
END
You'll obviously need to modify/simplify for your situation, but the basic context is there.