根据SQL Server中的条件从Count计数

Question

我的数据库中有4个表。

1. Students（Idno, Name, CourseId）

示例数据：

Idno    Name    CourseId
------------------------
-101123456  Vijay   101
-101123457  John    102
-101123458  Sam 101
-101123459  Arvind  102
-101123460  Smith   101

1. Courses（CourseId, CourseNo, CourseName, StreamId）

示例数据：

CourseId    CourseNo     CourseName StreamId
------------------------------------------
-101    53245   C   1
-102    53245   C++ 2

1. Streams（StreamId, StreamName）

示例数据：

StreamId    StreamName  
---------------------------
-1          Engineering
-2          Medical

1. Booking（BId, Idno, BStatus）

示例数据：

Bid     Idno        BStatus
--------------------------------
-1110   101123456   Confirmed
-1111   101123456   Confirmed
-1112   101123457   Confirmed
-1113   101123458   Confirmed
-1114   101123459   Confirmed
-1115   101123460   Confirmed
-1116   101123456   Confirmed
-1117   101123457   Confirmed
-1118   101123458   Confirmed
-1119   101123459   Confirmed
-1119   101123460   Cancelled

我在生成以下输出时遇到问题

SNo Stream        BookedOnce BookedTwice NonBooked
1    Engineering     2           3         0
2    Medical         3           1          1

由于

Answer 1

我认为这需要两个步骤。首先，计算每个学生对流的预订数量。然后按流，计算有一次，两次或零预订的学生人数。

以下是一个示例，内部查询的第一步是：

select  StreamId
,       StreamName
,       sum(case when Bookings = 1 then 1 else 0 end) as BookedOnce
,       sum(case when Bookings = 2 then 1 else 0 end) as BookedTwice
,       sum(case when Bookings = 0 then 1 else 0 end) as NoneBooked
from    (
        select  str.StreamId
        ,       str.StreamName
        ,       s.Idno
        ,       count(b.BId) as Bookings
        from    Students s
        left join
                Booking b
        on      b.Idno = s.Idno
        left join
                Courses c
        on      c.CourseId = s.CourseId
        left join
                Streams str
        on      str.StreamId = c.StreamId
        group by        
                str.StreamId
        ,       str.StreamName
        ,       s.Idno
        ) BookingsPerStudentPerStream
group by        
        StreamId
,       StreamName