Question

我正在做家庭作业问题

我需要轮询三张桌子并查找少花钱的学生的名字超过3.5 GPA，并且不到9学分

我已经想出如何填写少于9学分的学生的姓名，但我仍然坚持要求他们的学分。

学生表包含Student ID（主键），Name，Year（新鲜，soph等），Major和GPA。

课程表包含course ID（主键），Name of class和credit hour rating for that class。

注册表包含Student ID（使其成为外键），Course1（FK），Course2，Course3和Course4。

我不是在寻找'正确'的答案，而是一些方向感。

到目前为止，我的查询是

SELECT DISTINCT NAME 
From StudentTable,EnrollmentTable,CourseTable
WHERE (GPA <3.5)

我想在GPA检查后需要一些东西，分析每个学生的入学表，检查每门课程（Course1，Course2等），if！null，使用该课程ID并轮询课程表以查找学分该课程的价值。然后使用SUM检查是否＆lt; 9。

思考？感谢您的帮助！

Answer 1

如果数据库模式由您决定，那么我会稍微更改一下。注册表示学生和课程之间的映射。我将它简化为两列：StudentID和CourseID（如果您不希望表的复合主键，则可以添加第三列EnrollmentID）。

执行此操作后，您可以加入学生 - ＆gt;注册 - ＆gt;课程。按学生ID分组，并使用学生学分的总和上的HAVING子句过滤，并使用学生GPA上的WHERE过滤。

有意义吗？

Answer 2

您需要获得给定学生的学分时数总和，这应该引导您考虑一个小组

让我们把它分解。首先，您希望学生使用＆lt; 3.5 GPA，这很简单：

SELECT S.Id, S.Name
FROM Student S
WHERE S.GPA < 3.5

通过为注册添加多个列而不是每个学生的注册中的多行（这将更为正常），它变得有点棘手

就个人而言，我会将其转换为“标准化”视图：

SELECT StudentId, Course1 AS CourseId
FROM Enrollment E
UNION
SELECT StudentId, Course2 AS CourseId
FROM Enrollment E
UNION
SELECT StudentId, Course3 AS CourseId
FROM Enrollment E
UNION
SELECT StudentId, Course4 AS CourseId
FROM Enrollment E

将其与第一个查询相结合：

SELECT S.Id, S.Name
FROM Student S
JOIN
(
    SELECT StudentId, Course1 AS CourseId
    FROM Enrollment E
    UNION
    SELECT StudentId, Course2 AS CourseId
    FROM Enrollment E
    UNION
    SELECT StudentId, Course3 AS CourseId
    FROM Enrollment E
    UNION
    SELECT StudentId, Course4 AS CourseId
    FROM Enrollment E
) E
  ON E.StudentId = S.StudentId
WHERE S.GPA < 3.5

然后你需要实际课程来获得CreditHourRating：

SELECT S.Id, S.Name, C.CreditHourRating
FROM Student S
JOIN
(
    SELECT StudentId, Course1 AS CourseId
    FROM Enrollment E
    UNION
    SELECT StudentId, Course2 AS CourseId
    FROM Enrollment E
    UNION
    SELECT StudentId, Course3 AS CourseId
    FROM Enrollment E
    UNION
    SELECT StudentId, Course4 AS CourseId
    FROM Enrollment E
) E
  ON E.StudentId = S.StudentId
JOIN Course C
  ON E.CourseId = C.CourseId
WHERE S.GPA < 3.5

然后您只需要每个学生的总数，因此查询变为：

SELECT S.Id, S.Name, SUM(C.CreditHourRating)
FROM Student S
JOIN
(
    SELECT StudentId, Course1 AS CourseId
    FROM Enrollment E
    UNION
    SELECT StudentId, Course2 AS CourseId
    FROM Enrollment E
    UNION
    SELECT StudentId, Course3 AS CourseId
    FROM Enrollment E
    UNION
    SELECT StudentId, Course4 AS CourseId
    FROM Enrollment E
) E
  ON E.StudentId = S.StudentId
JOIN Course C
  ON E.CourseId = C.CourseId
WHERE S.GPA < 3.5
GROUP BY S.Id, S.Name

最后，你在CreditHoursRating上有条件：

SELECT S.Id, S.Name, SUM(C.CreditHourRating)
FROM Student S
JOIN
(
    SELECT StudentId, Course1 AS CourseId
    FROM Enrollment E
    UNION
    SELECT StudentId, Course2 AS CourseId
    FROM Enrollment E
    UNION
    SELECT StudentId, Course3 AS CourseId
    FROM Enrollment E
    UNION
    SELECT StudentId, Course4 AS CourseId
    FROM Enrollment E
) E
  ON E.StudentId = S.StudentId
JOIN Course C
  ON E.CourseId = C.CourseId
WHERE S.GPA < 3.5
GROUP BY S.Id, S.Name
HAVING SUM(C.CreditHourRating) < 9

构建具有多个WHERE要求的SQL查询，一个有点复杂，丢失

2 个答案: