Rollback transaction from trigger - sql

Question

In MS SQL Server 2008 R2, we want a pre-insert and pre-update trigger which checks something and allows or rollbacks (via raiserror) the running insert/update.
Question: In INSTEAD OF trigger. Does one really has to explicitly write the insert or update? Because we want the default insert or update to be done and only do the "precheck".

Answer 1

Yes.
You do need to write the explicit INSERT or UPDATE.
The trigger runs INSTEAD OF the DML operation. If you leave the trigger blank then no action will happen other than the INSERTED / DELETED tables being created and populated in tempdb.
Although from discussion in the comments I would not use a trigger for this at all but use a unique filtered index CREATE UNIQUE INDEX ix ON T(a,b,c) WHERE c <> ''. This is likely to be more performant and avoid potential logic issues when dealing with concurrency.

Answer 2

You probably do not want an INSTEAD OF trigger unless you want to replace the actual insert or update. In your case, you want a FOR INSERT, UPDATE trigger instead.
This example trigger prints a message to the client when anyone tries to add or change data in the titles table.
USE pubs
IF EXISTS (SELECT name FROM sysobjects
WHERE name = 'reminder' AND type = 'TR')
DROP TRIGGER reminder
GO
CREATE TRIGGER reminder
ON titles
FOR INSERT, UPDATE
AS RAISERROR ('inserts and updates to the titles table is not allowed', 16, 1)
GO
You could also use things like IF EXISTS or COLUMNS_UPDATED as well.
Here another example that uses rollback.
USE pubs
IF EXISTS (SELECT name FROM sysobjects
WHERE name = 'employee_insupd' AND type = 'TR')
DROP TRIGGER employee_insupd
GO
CREATE TRIGGER employee_insupd
ON employee
FOR INSERT, UPDATE
AS
/* Get the range of level for this job type from the jobs table. */
DECLARE #min_lvl tinyint,
#max_lvl tinyint,
#emp_lvl tinyint,
#job_id smallint
SELECT #min_lvl = min_lvl,
#max_lvl = max_lvl,
#emp_lvl = i.job_lvl,
#job_id = i.job_id
FROM employee e INNER JOIN inserted i ON e.emp_id = i.emp_id
JOIN jobs j ON j.job_id = i.job_id
IF (#job_id = 1) and (#emp_lvl <> 10)
BEGIN
RAISERROR ('Job id 1 expects the default level of 10.', 16, 1)
ROLLBACK TRANSACTION
END
ELSE
IF NOT (#emp_lvl BETWEEN #min_lvl AND #max_lvl)
BEGIN
RAISERROR ('The level for job_id:%d should be between %d and %d.',
16, 1, #job_id, #min_lvl, #max_lvl)
ROLLBACK TRANSACTION
END
I'm not sure if you have a transaction or not, but in your case you would want something like the following:
USE myDatabase
IF EXISTS (SELECT name FROM sysobjects
WHERE name = 'myTable' AND type = 'TR')
DROP TRIGGER tr_myTrigger
GO
CREATE TRIGGER tr_myTrigger
ON myTable
FOR INSERT, UPDATE
AS
if(exists(select * from inserted where rtrim(c) <> ''))
begin
-- check to make sure the insert(s) are unique
if(exists(
select * from inserted i
join dbo.myTable t on i.a = t.a and i.b = t.b and i.c = t.c)
begin
raiserror('Duplicate(s) found', 16, 1)
rollback transaction
end
end