Limit column value to 0 or to be unique on insert

Question

I have a table where a int column should either be set to zero or to a value which does not already exist in the table. Can I prevent inserting non zerod duplicated values in such column with a CHECK CONSTRAINT or should I use a BEFORE INSERT trigger? in case I could do this with both, what design is better?

From the .NET windows forms application we are using a global transaction scope to wrap the save and in both cases I would like the insert to fail and the transaction to roll back completely so I don't know if I should put the rollback inside the trigger, that's why I would rather try with a check if possible.

Database: SQL 2008

Thanks.

Answer 1

See the link in Andriy M's comment, it mention a 2008 new concept : filtered index...

CREATE UNIQUE INDEX indexName ON tableName(columns) INCLUDE includeColumns WHERE columnName != 0

This will create an index of unique items that are not 0.

Any attempt to insert a duplicate non-zero value will breach the uniqueness of the index and cause an error.

Answer 2

why are you using zero instead of null.? If you had it as null then the db would handle it for you easily via a nullable unique constraint..

Answer 3

Check constraint, when used properly, prevent bad data. They do not change the bad data to good. For that reason, I would aim for a trigger instead. If you can get around the need for a 0 as NULL, you could use a unique constraint, but supplying the answer would be the job of a trigger regardless.