选择每组中的最后一条记录以及计数

Question

我们有一个名为“atable”的以下 sqlite3 表

id  student assignment      grade
-----------------------------------
1    A       123             9
2    A       456             9
3    A       234             8
4    B       534             7
5    B       654             9
6    C       322             7

id 是唯一的，并且对于每条记录都会递增。我们正在通过运行查询获取每个用户的最新分配

SELECT student, assignment, grade from atable where id in 
       (select max(id) from atable group by student) order by id desc

这工作正常。但是，我们还需要获取每个用户在同一查询中获得特定成绩的作业数量，比如 9。

任何想法建议如何增强或重写上述查询以返回计数。如前所述，我们使用的是 sqlite3。

谢谢

Answer 1

您可以使用此相关查询：

SELECT t.student, t.assignment, t.grade, 
       (SELECT COUNT(*) FROM atable s
        WHERE s.student = t.student and s.grade >= 9) as total_above_9
from atable t
where t.id in 
   (select max(id) from atable group by student)
order by t.id desc

Answer 2

最好加入包含原始表的聚合版本的派生表：

select t1.student, t1.assignment, t1.grade, t2.cnt 
from mytable as t1
join (
   select student, max(id) as id, 
          count(case when grade = 9 then 1 end) as cnt
   from mytable 
   group by student
) as t2 on t1.id = t2.id

Answer 3

试试这个；）

select t1.student, t1.assignment, t1.grade, t2.count
from atable t1
inner join (select max(id) as id, count(if(grade=9, 1, null)) as count from atable group by student) t2
on t1.id = t2.id
order by t1.id desc