Hermione's reasoning is flawed. The wand chooses the wizard, not the other way around.

Yet another ill-described problem. The minimum cost for each wand is the cost of the wand, right? join to filter evil ones and sort and we're done. Yet, my answer is "incorrect". As I encounter more and more poorly worded problems, the less interested in this site I become. :(

Badly-written question. Nowhere is it made clear that it's looking for the cheapest wand for a given combination of age and power. I had to come to this discussion section to find that out.

worked in mysql

select w.id, p.age, w.coins_needed, w.power from Wands as w join Wands_Property as p on (w.code = p.code) where p.is_evil = 0 and w.coins_needed = (select min(coins_needed) from Wands as w1 join Wands_Property as p1 on (w1.code = p1.code) where w1.power = w.power and p1.age = p.age) order by w.power desc, p.age desc

My solution in SQL Server:

SELECT id, age, coins_needed, power
FROM 
(
    SELECT W.id, WP.age, W.coins_needed, W.power,
    ROW_NUMBER() OVER 
        (
            PARTITION BY W.code,W.power  
            ORDER BY W.coins_needed, W.power DESC
        ) AS RowNumber
    FROM Wands W WITH (NOLOCK)
    INNER JOIN Wands_Property WP WITH (NOLOCK) ON W.code = WP.code
    WHERE WP.is_evil = 0
)
AS Wand_Data
WHERE RowNumber = 1
ORDER BY power DESC, age DESC