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.