使用多个WHERE需求构建SQL查询，其中一个有点复杂，丢失了

我正在做一个家庭作业，在哪里

我需要调查三个表格，找出那些分数较低的学生的名字 平均成绩低于3.5分，且学时不足9学时

我已经知道了如何填充少于9个学时的学生的名字，但我一直在计算他们的学时

学生表包含学生ID主键、姓名、新学年、SOP等、专业和GPA

课程表包含课程ID主键、课程名称以及该课程的学分等级

注册表包含使其成为外键的学生ID、Course1 FK、Course2、Course3和Course4

我不是在寻找“正确”的答案，而是在寻找某种方向感

到目前为止，我的问题是

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

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我想在GPA检查之后，我需要一些东西来分析每个学生的报名表，检查每门课程1、2等等，如果有的话！null，使用该courseID并轮询课程表以查找该课程的学分值。然后使用SUM检查是否如果DB模式由您决定，那么我将对其进行一些更改。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。如果您不想为表添加复合主键，可以添加第三列EnrollmentID

完成此操作后，您可以加入学生->注册->课程。按StudentID分组，使用HAVING子句筛选学生的总学分，并使用WHERE on Student GPA筛选

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有意义吗？

如果DB模式由您决定，那么我会对其进行一些更改。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。如果您不想为表添加复合主键，可以添加第三列EnrollmentID

完成此操作后，您可以加入学生->注册->课程。按StudentID分组，使用HAVING子句筛选学生的总学分，并使用WHERE on Student GPA筛选

---

有意义吗？

你需要得到一个给定学生的总学时，这应该引导你考虑一组学生

让我们把它分解一下。首先，你希望学生的平均成绩低于3.5分，这很简单，只是：

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

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，您需要实际的课程来获得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，因此查询变成：

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，你有信用小时评级的条件：

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

---

你需要得到一个给定学生的总学时，这将引导你考虑一组学生

让我们把它分解一下。首先，你希望学生的平均成绩低于3.5分，这很简单，只是：

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

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，您需要实际的课程来获得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，因此查询变成：

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，你有信用小时评级的条件：

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

---

很有意思，但不幸的是桌子是给我们的。我从未听说过HAVE子句，所以我现在正在研究它。在这种情况下，查看注册和课程之间的左联接，并对每个注册->课程关系的ISNULLCourse.CreditHour，0求和。您将得到四个左连接，它们表示每个注册->课程关系。此外，HAVING子句用于根据感兴趣的聚合函数过滤结果集，但不幸的是，表是提供给我们的。我从未听说过HAVE子句，所以我现在正在研究它。在这种情况下，查看注册和课程之间的左联接，并对每个注册->课程关系的ISNULLCourse.CreditHour，0求和。您将得到四个左连接，它们表示每个注册->课程关系。此外，HAVING子句用于根据聚合函数过滤结果集，感谢您像Fergus一样对其进行分解！！！！！工作非常完美，我现在完全理解了实现。再次感谢！谢谢你像费格斯那样把它拆了！！！！！工作非常完美，我现在完全理解了实现。再次感谢！