关于mysql用户变量的使用问题

Question

题目如下，我想请问下，为什么得出的结果为null，就算查询不正确，按道理不应该为null才对啊，求大佬帮忙解答下

SELECT gender,`day`,
CASE @man := 0 AND @women := 0 WHEN gender = 'F' THEN @man := @man + score_points
ELSE @women := @women + score_points
END AS total 
FROM Scores
ORDER BY gender,day

结果：

题目如下：

Answer 1

其实可以用group by，类似如下试试

SELECT gender,`day`,sum(score_points)
FROM Scores
GROUP BY gender,day
ORDER BY gender,day