题目链接：​

题目大意：略。

解题思路：略。

AC 代码

-- 解决方案(1)
SELECT sale_date,
       SUM(CASE WHEN fruit='apples' THEN sold_num ELSE -sold_num END) AS diff
  FROM Sales
 GROUP BY sale_date
 ORDER BY sale_date

-- 解决方案(2)
SELECT sale_date, SUM(IF(fruit = 'apples', sold_num, 0)) - SUM(IF(fruit = 'oranges', sold_num, 0)) diff
FROM Sales
GROUP BY sale_date