How to reuse a sub query in sql?

Question

I have query like the following

select columns
from (select columns1
      from result_set
      where condition_common and condition1) as subset1
      join         


        

Answer 1

Use a Common Table Expression (CTE) if you're using SQL Server 2005+:

with cte as (
      select columns
      from result_set
      where condition_common
    )
select columns
from cte  as subset1
      join
      cte as subset2 
         on subset1.somekey = subset2.somekey
where otherconditions