Unique constraint vs pre checking

I use SQL Server 2008, and I have a table with a column of type varchar(X) which I want to have unique values.

What is the best way to achieve that? Should I use unique cons开发者_JAVA技巧traint and catch an exception, or should I pre-check before inserting a new value?

One issue, the application is used by many users so I guess that pre-checking might result in race condition, in case that two users will insert the same values.

Thanks

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Race condition is an excellent point to be aware of.

Why not do both? - pre-check so you can give good feedback to the user, but definitely have the unique constraint as your ultimate safeguard.

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Let the DB do the work for you. Create the unique constraint.

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If it's a requirement that the values be unique --- then a constraint is the only guaranteed way to achieve that. reliable so-called pre-checking will require a level of locking that will make that part of your system essentially single user.

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Use a constraint (UNIQUE or PRIMARY KEY). That way the key is enforced for every application. You could perform additional checks and handling in a store procedure if you need to - either before or after the insert.