MySQLクエリSelect、SUM、LEFT JOIN

これを試してください：

SELECT S.Items as Products, 
       COALESCE(PS.Quantity, 0) as [Products Sold], 
       COALESCE(PP.Quantity, 0) as [Products purchased]
From Stock S 
LEFT JOIN (SELECT Items, SUM(Quantity) AS Quantity 
           FROM products_purchased 
           GROUP BY Items
         ) AS PP ON S.Items = PP.Items 
LEFT JOIN (SELECT Items, SUM(Quantity) AS Quantity 
           FROM products_sold 
           GROUP BY Items
         ) AS PS ON S.Items = PS.Items;