JOINS, EXISTS or IN which is better? Few questions on SQL

Question

I have few questions on SQL..

1. How to analyze the performance of a query? Any software, inbuilt features of MSSQL server 2005/2008?

2. What shou

Answer 1

1. As others have said, check the "execution plan". SQL Server Management studio can show you two kinds of execution plans, estimated and actual. Estimated is how SQL Server guesses it would execute the query and is returned without actually executing the query, and the actual plan is returned together with a result set and shows what was actually done.

2. That query looks good, but you have to make sure that you have an index on enquiry_courses.enquiry_id, and it's probably best that enquiries.enquiry_id is not nullable.

3. The semantics of IN and EXISTS are slightly different (IN will return no rows if there is one or more NULLs in the subquery). If the subquery is guaranteed to be not null, it doesn't matter. There is some kind of "internet truth" that you should use EXISTS on SQL Server and IN on Oracle, but this might have been true when dinosaurs ruled the planet but it doesn't apply anymore. IN and EXISTS both perform a semi-join, and the optimizer is more than capable of deciding how to execute this join.