Question

我正在尝试编写一个计算并列出学生GPA的查询，然后只显示低于所有学生平均GPA的查询。我已成功获得查询以输出包含每个学生的GPA的表格，但我无法仅显示低于平均水平的表格。

以下是我目前的查询：

    SELECT SC.StudentID, S.StdLastName, GPA
    FROM (SELECT SC.StudentID, S.StdLastName, ROUND(SUM(C.Credits * GV.GradeValue)/SUM(C.Credits), 2) AS GPA
         FROM ((Student_Classes AS SC INNER JOIN GradeValues AS GV ON SC.Grade = GV.Grade) 
         INNER JOIN Students AS S ON SC.StudentID = S.StudentID)
         INNER JOIN Classes as C ON SC.ClassID = C.ClassID
         GROUP BY SC.StudentID, S.StdLastName)
    GROUP BY SC.StudentID, S.StdLastName, GPA
    HAVING GPA < AVG(GPA)
    ORDER BY GPA;

当我这样做时，出现的错误是，

您尝试执行的查询不包含指定的表达式GPA＆lt; AVG（GPA）作为聚合函数的一部分。

我在此网站上发现了类似错误的多个网页，但仍无法使其正常运行。任何帮助将不胜感激。

Answer 1

当您说GROUP BY SC.StudentID, S.StdLastName, GPA时，您告诉数据库使用这三列创建您的数据组，然后在每个组中应用聚合函数（在本例中为AVG）。这意味着，正如所写，您试图根据该学生的平均值过滤每个学生的GPA。希望每个学生的平均GPA等于他们的实际GPA ......

为了使这项工作，您需要第二个子查询，您可以使用它来获得总体平均GPA：

SELECT sc.studentid, s.stdlastname, gpa
FROM   (SELECT sc.studentid, 
               s.stdlastname, 
               ROUND(SUM(c.credits * gv.gradevalue) / SUM(c.credits), 2) AS gpa
        FROM      ((student_classes sc
              INNER JOIN gradevalues gv
                 ON sc.grade = gv.grade)
              INNER JOIN students s
                 ON sc.studentid = s.studentid)
        GROUP BY sc.studentid, s.stdlastname)
GROUP BY sc.studentid, s.stdlastname, gpa
WHERE gpa < (SELECT AVG(GPA) as avg_gpa 
               FROM (SELECT ROUND(SUM(c.credits * gv.gradevalue) / SUM(c.credits), 2) AS gpa
                     FROM  student_classes sc
                           INNER JOIN gradevalues gv
                                   ON sc.grade = gv.grade
                     GROUP BY sc.studentid))
ORDER BY gpa;

我不确定这是否会解决你的语法问题，但它应该解决你的逻辑问题。

Answer 2

这是否可以消除错误？如果是，您只需从此

中选择sid，lname和GPA即可

  SELECT SC.StudentID, S.StdLastName, GPA, avg(GPA) as avgGPA
FROM (SELECT SC.StudentID, S.StdLastName, ROUND(SUM(C.Credits * GV.GradeValue)/SUM(C.Credits), 2) AS GPA
     FROM ((Student_Classes AS SC INNER JOIN GradeValues AS GV ON SC.Grade = GV.Grade) 
     INNER JOIN Students AS S ON SC.StudentID = S.StudentID)
     INNER JOIN Classes as C ON SC.ClassID = C.ClassID
     GROUP BY SC.StudentID, S.StdLastName)
WHERE GPA < avg(GPA)
GROUP BY SC.StudentID, S.StdLastName, GPA
    ORDER BY GPA;

SQL - 不包括指定的表达式作为聚合函数

2 个答案: