如何选择一组最小值的行，同时还使用where子句 [英] How to select the row of a minimum value of a group, while also using a where clause

问题描述

我有一张桌子，该桌子可以跟踪课程的出勤情况.列是课程编号，课程，人员编号和日期.我有一个查询(如下)，该查询提取一个人出现的最早日期以及相关的课程，课程和人格信息.这用于确定某人何时开始特定课程，并确保他们从第一堂课开始.这工作正常，但我遇到的问题是每个课程都在运行此查询.例如，找到每个人在特定课程中的开始日期，而不是每个课程的开始日期.现在，我只是在运行更通用的查询并在biz层中对其进行过滤.

I have a table that tracks attendance in a course. The columns are the courseid, lesson, personid, and date. I have a query (below) that extracts the earliest date a person appears along with the associated course, lesson, and personid. This is used to determine when a person started a particular course and ensure they started with the first lesson. This works fine, but where I am stuck is running this query per course. For example, finding the first date each person in a particular course started it rather than for every course. Right now I am just running the more general query and filtering it in the biz layer.

我对此感到困惑，因此请原谅任何错别字:

I obfuscated this a bit so forgive any typos:

select a.courseid, 
       a.lesson, 
       a.personid, 
       a.thedate 
from (select personid, 
             min(thedate) as earliestdate 
      from attendance 
      group by personid) as x 
inner join attendance as a on (a.personid = x.personid and a.thedate = x.thedate)

推荐答案

仅对person_id进行分组，当然在内部查询中:

Just group over person_id, course in the inner query:

select a.courseid, a.lesson, a.personid, a.thedate 
from (
    select personid, courseid, min(thedate) as earliestdate 
    from attendance 
    group by personid, courseid
) as x 
inner join attendance as a 
  on (a.personid = x.personid and 
      a.thedate = x.thedate and 
      a.courseid=x.course_id)

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