https://leetcode-cn.com/problems/rank-scores/
178. 分数排名
select s1.Score, count(distinct s2.Score) as Rank from scores as s1, scores as s2 where s1.Score <= s2.Score group by s1.Id order by s1.Score desc;
具体思路就是把一个表变成两个表，然后进行比较
select s1.Score, count(s2.Score) as Rank from Scores as s1, Scores as s2 where s1.Score <= s2.Score group by s1.Id order by s1.Score desc;
select * from Scores as s1, Scores as s2;
#执行顺序：
#from > on > join > where > group by > having > select > distinct > order by > limit
#第一步，from对两个表计算笛卡尔积
#第二步，where判断小于等于条件，进行筛选
#第三步，group by进行分组去重，这样，s2.scores多行就会被压在一行中
#第四步，distinct计算不同的个数
#第五步，order by进行排序
具体说一下为什么要用group by ，如果不用group by的话，原先的数据集是一个大组，只会对一行进行处理。而group by之后，是分成多个小组，然后每一个小组分别取进行处理