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Never use the sp_ prefix for a user-stored procedure. One reason is that it can make it hard to tell which sprocs are delivered and which are user-defined. Another is that if you prefix your sprocs with sp_, SQL searches for the prefix in the master db each time the procedure is called, before looking in the local database. Use usp_ instead.

From Microsoft eLearning Course 3595: Establishing Database Conventions and Standards for Microsoft® SQL Server™ 2005.

The EXISTS and NOT EXISTS operators can often use an index whereas the LIKE and NOT LIKE operators cause a table scan. This makes EXISTS and NOT EXISTS tend to be more efficient.