Question

I’m sure this is a common request but I wouldn’t know how to ask for it formally.

I encountered this a long time ago when I was in the Army. A soldier has multiple physical fitness tests but the primary test that counts in the most recent. The soldier also has multiple marksmanship qualifications but only the most recent qualification to the weapon assigned is significant.

How do you create a view that itemizes the most significant child of the parent?

Answer 1

Use:

  SELECT p.*, x.*
    FROM PARENT p
    JOIN CHILD x ON x.parent_id = p.id
    JOIN (SELECT c.id,
                 c.parent_id,
                 MAX(c.date_column) AS max_date
            FROM CHILD c
        GROUP BY c.id, c.parent_id) y ON y.id = x.id
                                     AND y.parent_id = x.parent_id
                                     AND y.max_date = x.date

Assuming SQL Server 2005+:

WITH summary AS (
     SELECT p.*, 
            c.*, 
            ROW_NUMBER() OVER (PARTITION BY p.id 
                                  ORDER BY c.date DESC) AS rank
       FROM PARENT p
       JOIN CHILD c ON c.parent_id = p.id)
SELECT s.*
  FROM summary s
 WHERE s.rank = 1