SQL Server 替代在聚合函数中使用子查询

Question

我正在使用聚合函数来计算特定学生的 GPA。我有三个表：COURSE、SECTION 和 ENROLL。这些表是这样创建的：

create table COURSE
(
    Cno     varchar(9) primary key, 
    Cname   varchar(50),
    Credit  int check (Credit > 0)
);

create table SECTION
(
    Cno     varchar(9) REFERENCES COURSE(cno),
    Sno     varchar(9),
    Semester    varchar(15) check(Semester in('Fall','Spring','Summer')), 
    Year    int, 
    Sid     varchar(9) primary key 
);

create table ENROLL
(
    Mno     varchar(9) REFERENCES STUDENT(Mno),
    Sid     varchar(9) REFERENCES SECTION(Sid),
    Grade   CHAR check(Grade in('A','B','C','D','F')),
    primary key(Mno,Sid)
);

我必须将 ENROLL 表中的 char Grade 转换为浮点值。在我的情况下计算 GPA 的方法是：

SUM(grade of each course*credit hours of the course)/SUM(credit hours of each course)

这是我目前尝试计算 GPA 的方式：

 @grade =     
(
 (select SUM(CASE 
              WHEN Grade = 'A' THEN 4.0
              WHEN Grade = 'B' THEN 3.0
              WHEN Grade = 'C' THEN 2.0
              WHEN Grade = 'D' THEN 1.0
              WHEN Grade = 'F' THEN 0.0
             END
             * (
                select c.Credit from COURSE c where c.Cno = (
                 select s.Cno from SECTION s where s.Sid in (
                  select Sid from ENROLL where Mno = @mNum
                 )
                )
             )
         ) FROM ENROLL 
         where Mno = @mNum 
 ) / (
  select SUM(Credit) from COURSE c where c.Cno = (
   select s.Cno from SECTION s where s.Sid in (
    select Sid from ENROLL where Mno = @mNum
   )
  ) group by c.Cno
 )
)

然而，有一个明显的问题。聚合函数中不能有子查询，所以我很难找到替代方法。任何帮助将不胜感激。

Answer 1

看来你只需要把三个表一起JOIN：

SELECT ENROLL.Mno, SUM(CASE
    WHEN Grade = 'A' THEN 4.0
    WHEN Grade = 'B' THEN 3.0
    WHEN Grade = 'C' THEN 2.0
    WHEN Grade = 'D' THEN 1.0
    WHEN Grade = 'F' THEN 0.0
END * COURSE.Credit) / SUM(COURSE.Credit)
FROM ENROLL
INNER JOIN SECTION ON ENROLL.Sid = Section.Sid
INNER JOIN COURSE ON SECTION.Cno = COURSE.Cno
GROUP BY ENROLL.Mno

Answer 2

我会尝试先加入 TABLES ENROLL 和 SECTION。

这将为我提供与 Mno 相关的所有部分。

之后我将加入 COURSE 表以获取与 Mno 关联的所有课程。

选择 * 从报名 加入b部分 a.Sid=b.Sid 加入课程 c 关于 b.Cno=c.Cno

然后我将按 a.Mno 分组。这意味着我正在根据定义的计算计算每个 Mno 的 GPA。

**SUM(grade of each course*credit hours of the course)/SUM(credit hours of each course)**

select a.Mno
       ,sum(case WHEN c.Grade = 'A' THEN 4.0
                 WHEN c.Grade = 'B' THEN 3.0
                 WHEN c.Grade = 'C' THEN 2.0
                 WHEN c.Grade = 'D' THEN 1.0
                 WHEN c.Grade = 'F' THEN 0.0
             END * c.Credit)/sum(c.credit) 
  from enroll a
  join section b
    on a.Sid=b.Sid
  join course c
    on b.Cno=c.Cno
group by a.Mno