如何选择组的最小值行,同时还使用where子句

Question

我有一张桌子可以跟踪课程的出勤情况.列是课程,课程,人格和日期.我有一个查询(下面),它提取一个人出现的最早日期以及相关的课程,课程和人格.这用于确定一个人何时开始特定课程并确保他们从第一课开始.这工作正常,但我遇到的问题是每个课程运行此查询.例如,找到特定课程中每个人的第一个日期,而不是每个课程.现在我只是运行更通用的查询并在biz层中过滤它.

我混淆了这一点,原谅任何错别字:

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)

Answer 1

只需将person_id分组,当然在内部查询中:

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)