如何根据唯一列和单独列中的最高量过滤结果集？

Question

标题可能令人困惑。不确定如何将我的问题用语言表达出来。这是我正在使用的 SQL 语句：

SELECT * FROM (
   select * from course_student
   order by cnum ASC,
            year desc,
            case semester
               when 'Spring' then 1
               when 'Summer' then 2
               when 'Fall' then 3
            end DESC
   ) AS example
WHERE example.sid = 1
AND example.grade != 'I';

这是该查询的结果：

SID  CNUM       GRADE  SEMESTER  YEAR
1    "CPS441"   "A"   "Fall"     2013
1    "CPS441"   "D"   "Spring"   2012
1    "CPS442"   "B"   "Summer"   2013
1    "CPS445"   "A"   "Spring"   2013

基本上，我正在抓住特定学生上过的每一门课。在这一点上，我想修剪第一次出现之后的每个重复结果（因为最高结果将是该课程的最新尝试）。所以在这种情况下，我希望从结果集中过滤掉第二个等级为“D”的 CPS441 类，并留下以下内容：

SID  CNUM       GRADE  SEMESTER  YEAR
1    "CPS441"   "A"   "Fall"     2013
1    "CPS442"   "B"   "Summer"   2013
1    "CPS445"   "A"   "Spring"   2013

另一个例子是，如果我从这个开始：

SID  CNUM       GRADE  SEMESTER  YEAR
1    "CPS441"   "A"   "Fall"     2013
1    "CPS441"   "D"   "Spring"   2012
1    "CPS442"   "B"   "Summer"   2013
1    "CPS442"   "C"   "Spring"   2013
1    "CPS445"   "A"   "Spring"   2013

我想这样结束：

SID  CNUM       GRADE  SEMESTER  YEAR
1    "CPS441"   "A"   "Fall"     2013
1    "CPS442"   "B"   "Summer"   2013
1    "CPS445"   "A"   "Spring"   2013

Answer 1

DISTINCT ON的教科书案例：

SELECT DISTINCT ON (cnum) *
FROM   course_student
ORDER  BY cnum
        , year DESC
        , CASE semester
            WHEN 'Fall'   THEN 1
            WHEN 'Summer' THEN 2
            WHEN 'Spring' THEN 3
          END
WHERE  sid = 1
AND    grade <> 'I';

见：

• Select first row in each GROUP BY group?

有一点不清楚： 如果最近的事件没有sid = 1 AND grade <> 'I'，您宁愿将学生从结果中完全删除吗？或者只是删除这些行并为同一个学生获取第一个有效行（就像我的解决方案一样）？

无论哪种方式，没有有效行的学生都不在结果中。

Answer 2

你似乎想要row_number()：

select * 
from (select cs.*,
             row_number() over (partition by sid, cnum
                                order by year desc,
                                         case semester when 'Spring' then 1 when 'Summer' then 2 when 'Fall' then 3 end desc
                               ) as seqnum 
      from course_student
      where grade <> 'I' and sid = 1
     ) cs
where seqnum = 1;