同じ列と単一のMySQLクエリで2つの別々の日付範囲の値の合計を取得する

これを試してください....

select a.date,sum(b.qty * b.price) As sum from sales As a, product As b where DATE_FORMAT(a.date,'%d-%m-%Y') between '27-05-2014' AND '28-05-2014' and a.id_sale=b.id_sale group by a.date