Overall, how you write your Transact-SQL (T-SQL) code can have one of the greatest impacts on your SQL Server performance. Regardless of how well you’ve optimized your server configuration and database design, poorly written and inefficient SQL code still results in poor performance. The following sections list some general guidelines to help you write efficient, faster-performing code.
General T-SQL Coding Guidelines
IF EXISTS instead of
SELECT COUNT(*) when checking only for the existence of any matching data values.
IF EXISTS stops the processing of the
SELECT query as soon as the first matching row is found, whereas
SELECT COUNT(*) continues searching until all matches are found, wasting I/O and CPU cycles. ...