mysql - 统计上个月 SQL 的记录数

Question

你能帮我完成一个简单的任务，但我真的不知道我应该如何完成这个请求。请帮助。

我有:

doctors
id name             profession 
 1 James Harden     dental 
 2 James Jones      therapist 
 3 LeBron James     cardiologist 
 4 Kobe Braynt      surgeon 
 5 Sabrina Williams nurse 
 6 Tyler Okonma     speech therapist 

patients 
id name         diagnostic 
 1 Mo Bamba     tooth pulling out
 2 Kaney West   astma 
 3 Post Malone  heart attack 
 4 Denzel Curry headache 
 5 Nicola Jokic stomac-ache 
 6 Dwayne Wade  AIDS 

visits
doctorId patientId visitDate 
       1         1 2019-03-09
       2         4 2019-03-01
       2         5 2019-02-26
       2         6 2019-02-05
       3         3 2019-03-03
       4         2 2019-03-07

我需要显示上个月为最多患者提供服务的医生。我无法计算出由医生服务的病人数量。您能建议一下这个请求吗？

Answer 1

您需要group by doctorid visits表并加入 doctors表:

select d.name, g.counter
from doctors d inner join (
  select doctorid, count(distinct patientid) counter
  from visits
  where 
    year(visitdate) = year(current_date - interval 1 month)
    and 
    month(visitdate) = month(current_date - interval 1 month)
  group by doctorid
  order by counter desc limit 3
) g on g.doctorid = d.id
order by g.counter desc, d.name 

您可以更改limit 3到你喜欢的。