Medium: Question 7

/*Question 7: Medium Part*/
SELECT w.ID,
       wp.age,
       w.coins_needed,
       w.POWER
FROM wands w,
     wands_property wp
WHERE w.code = wp.code
  AND w.coins_needed =
    (SELECT MIN(w2.coins_needed)
     FROM wands w2,
          wands_property wp2
     WHERE w2.code = wp2.code
       AND w.POWER = w2.POWER
       AND wp.age = wp2.age
       AND wp2.is_evil = 0)
ORDER BY w.POWER DESC,
         wp.age DESC;