Self-join with nested select query in T-SQL

Question

Is it possible to join a nested select statement with itself (without writing it out twice and running it twice)

Something like this would be ideal

SELECT P.Child, P.Parent, Q.Parent AS GrandParent
FROM (SELECT Child, Parent FROM something-complex) AS P
LEFT JOIN P AS Q ON Q.Child = P.Parent

Answer 1

50% possible. You can use a CTE to avoid writing it twice but it will still execute twice.

;WITH p
     AS (SELECT child,
                parent
         FROM   something-complex)
SELECT p.child,
       p.parent,
       q.parent AS grandparent
FROM   p
       LEFT JOIN p AS q
         ON q.child = p.parent  

If the query is expensive you would need to materialize it into a table variable or #temp table to avoid the self join causing two invocations of the underlying query.

Answer 2

You could use a common table expression:

WITH P AS (SELECT Child, Parent FROM something-complex)
SELECT P.Child, P.Parent, Q.Parent as GrandParent
LEFT JOIN P AS Q ON Q.Child = P.Parent