-- 16. 按季度统计销售量SELECT YEAR(订单.订购日期) AS 年度, (CASE WHEN MONTH(订单.订购日期) IN (1,2,3) THEN '1' WHEN MONTH(订单.订购日期) IN (4,5,6) THEN '2' WHEN MONTH(订单.订购日期) IN (7,8,9) THEN '3' ELSE '4' END) AS 季度,SUM(订单明细.数量) AS 订购数量FROM 订单LEFT JOIN 订单明细 ON 订单明细.订单ID = 订单.订单ID GROUP BY 年度,季度 ORDER BY 年度,季度结果如下: