SQL - 从多个表中选择计数

Question

这是我的问题（我正在使用SQL Server）

我有一个表Students (StudentId, Firstname, Lastname, etc)。

我有记录StudentAttendance (StudentId, ClassDate, etc.)

我记录其他学生活动（我在这里概括为简单起见），如表Papers一个(StudentId, PaperId, etc.)表。可能有从0到20个文件转入的任何地方。同样，有一个表Projects (StudentId, ProjectId, etc.)。与Papers相同。

我想要做的是为参加超过一定水平的学生（例如10名出勤）创建一个计数列表。事情是这样的：

ID Name  Att Paper Proj 
123 Baker  23  0  2 
234 Charlie 26  5  3 
345 Delta  13  3  0 

以下是我有：

select 
    s.StudentId, 
    s.Lastname, 
    COUNT(sa.StudentId) as CountofAttendance, 
    COUNT(p.StudentId) as CountofPapers 
from Student s 
inner join StudentAttendance sa on (s.StudentId = sa.StudentId) 
left outer join Paper p on (s.StudentId = p.StudentId) 
group by s.StudentId, s.Lastname 
Having COUNT(sa.StudentId) > 10 
order by CountofAttendance 

如果CountofPaper和join（无论是内部或左外）的Papers表被注释掉，查询工作正常。我收到了至少参加过10节课的学生。

但是，如果我把CountofPapers和加入，事情变得疯狂。通过左外连接，任何有论文的学生只会在论文栏中显示出席人数。通过内部联合，出勤率和纸张计数似乎可以相互叠加。

指导需要和赞赏。

戴夫

Answer 1

问题是有多个papers每个学生，所以StudentAttendance一行的Paper每一行加入：计数将被重新添加每次。试试这个：

select 
    s.StudentId, 
    s.Lastname, 
    (select COUNT(*) from StudentAttendance where s.StudentId = sa.StudentId) as CountofAttendance, 
    (select COUNT(*) from Paper where s.StudentId = p.StudentId) as CountofPapers 
from Student s 
where (select COUNT(*) from StudentAttendance where s.StudentId = sa.StudentId) > 10 
order by CountofAttendance 

编辑过参考合并问题CountofAttendance

顺便说一句，这不是最快的解决方案，但它是最容易理解的，这是我的本意。您可以通过使用加入别名选择来避免重新计算，但正如我所说的，这是最简单的。

Answer 2

试试这个：

select std.StudentId, std.Lastname, att.AttCount, pap.PaperCount, prj.ProjCount 
from Students std 
    left join 
    (
     select StudentId, count(*) AttCount 
     from StudentAttendance 
    ) att on 
     std.StudentId = att.StudentId 
    left join 
    (
     select StudentId, count(*) PaperCount 
     from Papers 
    ) pap on 
     std.StudentId = pap.StudentId 
    left join 
    (
     select StudentId, count(*) ProjCount 
     from Projects 
    ) prj on 
     std.StudentId = prj.StudentId 
where att.AttCount > 10 

Answer 3

看看使用Common Table Expressions，然后分而治之，解决您的问题。顺便说一句，你是关闭的1原始查询，您将有11最低勤

; 
WITH GOOD_STUDENTS AS 
(
-- this query defines all students with 10+ attendance 
SELECT 
    S.StudentID 
, count(1) AS attendence_count 
FROM 
    Student S 
    inner join 
    StudentAttendance sa 
    on (s.StudentId = sa.StudentId) 
GROUP BY 
    S.StudentId 
HAVING 
    COUNT(1) >= 10 
) 
, STUDIOUS_STUDENTS AS 
(
-- lather, rinse, repeat for other metrics 
SELECT 
    S.StudentID 
, count(1) AS paper_count 
FROM 
    Student S 
    inner join 
    Papers P 
    on (s.StudentId = P.StudentId) 
GROUP BY 
    S.StudentId 
) 
, GREGARIOUS_STUDENTS AS 
(
SELECT 
    S.StudentID 
, count(1) AS project_count 
FROM 
    Student S 
    inner join 
    Projects P 
    on (s.StudentId = P.StudentId) 
GROUP BY 
    S.StudentId 
) 
-- And now we roll it all together 
SELECT 
    S.* 
, G.attendance_count 
, SS.paper_count 
, GS.project_count 
-- ad nauseum 
FROM 
    -- back to the well on this one as there may be 
    -- students did nothing 
    Students S 
    LEFT OUTER JOIN 
     GOOD_STUDENTS G 
     ON G.studentId = S.studentId 
    LEFT OUTER JOIN 
     STUDIOUS_STUDENTS SS 
     ON SS.studentId = S.studentId 
    LEFT OUTER JOIN 
     GREGARIOUS_STUDENTS GS 
     ON GS.studentId = S.studentId 

我看到很多其他的答案滚滚而来，但我打的时间太长了退出;）