查詢需求 ： 分組查出銷售表中所有會員購買金額，同時分組查出退貨表中所有會員的退貨金額，把會員id相同的購買金額-退款金額得到的結果更新到表會員表中對應會員的積分欄位（credits）

SELECT
	s.memberid,
	s.y1,
	s1.y2,
	(s.y1 - s1.y2)
FROM
	(
		SELECT
			y.memberid,
			sum(y.MNAccount) y1
		FROM
			member d
		LEFT JOIN sale y ON d.memberid = y.memberid
		GROUP BY
			y.memberid
	) s
LEFT JOIN (
	SELECT
		y.memberid,
		sum(y.RMNAccount) y2
	FROM
		member d
	LEFT JOIN regoods y ON d.memberid = y.memberid
	GROUP BY
		y.memberid
) s1 ON s.memberid = s1.memberid