Question

What does a Transact SQL statement Select statement inside a from mean?

I mean something like this

 .. from ( 
    select .. 
 )

Also, I need to know if the statement is bad for performance. Can you provide me a link to the official documentation about this topic in Transact SQL?

Answer 1

I think you are talking about subquery. A subquery is used to return data that will be used in the main query as a condition to further restrict the data to be retrieved.

Please refer this link:- http://www.tutorialspoint.com/sql/sql-sub-queries.htm