Rownum, Order by, inner-inner query

Question

I have a query where the current approach to handling rownum and order by's doesn't work and was wondering if anyone knew of a way around this. The basic query is as follows:
Select *
From <list of tables> T1
Where....
And t1.elig_id =
(Select elig_id
From (Select elig_id
From eligibility T2
where T1.family_number = T2.family_number
order by date1 desc, date2 desc)
Where rownum = 1)
The problem is the T1.family_number = T2.family_number line. Since the join is in the inner-inner query sql does not recognize T1.family_number. Is there anyway around that?

Answer 1

It is probably not materializing the sub query for every row in the outer select, but IN can be expensive if the sub-query returns a large number of rows. Another way to phrase the statement which may be faster is:
SELECT t1.*
FROM <list of tables> T1,
     (SELECT elig_id
      FROM (SELECT elig_id, ROW_NUMBER() OVER (PARTITION BY family_number
                                           ORDER BY date1 DESC date2 DESC) rn
            FROM eligibility)
      WHERE rn = 1) elig
Where t1.elig_id = elig.elig_id and
      other_conditionsEssentially, use the sub-query as if it were a table (an in-line view actually), and join to it. This may make Oracle materialize it earlier and avoid the inlist processing.
HTH
John