筛选出每个月，门诊就诊人次前5的疾病。
Rank() over (partition by……order by……）的用法与效果：
SELECT *
from
(SELECT a.`医疗机构名称`,a.时间,a.疾病组名称,a.门诊就诊人次,Rank() over (partition by a.`医疗机构名称`,a.时间 order by a.门诊就诊人次 desc) as totalfee_num
from 0804renmin a) b
where totalfee_num <=5;
dense_rank() over (partition by……order by……）的用法与效果：
SELECT *
from
(SELECT a.`医疗机构名称`,a.时间,a.疾病组名称,a.门诊就诊人次,dense_rank() over (partition by a.`医疗机构名称`,a.时间 order by a.门诊就诊人次 desc) as totalfee_num
from 0804renmin a) b
where totalfee_num <=5;