是否可以将两个SQL（MS Access）查询合并为一个？

Question

我想知道是否有可能将两个查询合并为一个？我有两个SQL查询。第一年输出学生的平均最终成绩。第二个输出3年级学生的平均最终成绩。

我尝试过使用UNION但不幸的是，它只是将所有内容都转移到了一列。我通过MS Access使用SQL。

第一次查询：

SELECT Marks.Student_ID AS Year, SUM(Final_Mark)/COUNT(Marks.Student_ID) AS Year_1_Marks
FROM Marks, Students
WHERE Marks.Year_Taken = 1 AND Students.Student_ID IN
(SELECT Marks.Student_ID
FROM Marks, Students
WHERE Students.Student_ID = Marks.Student_ID) 
GROUP BY Marks.Student_ID

第二次查询：

SELECT Marks.Student_ID AS Year, SUM(Final_Mark)/COUNT(Marks.Student_ID) AS Year_3_Marks
FROM Marks, Students
WHERE Marks.Year_Taken = 3 AND Students.Student_ID IN
(SELECT Marks.Student_ID
FROM Marks, Students
WHERE Students.Student_ID = Marks.Student_ID) 
GROUP BY Marks.Student_ID

我想将输出分为3列，分别是Student_ID，Year_1_Marks，Year_3_Marks。任何帮助都非常感谢。

Answer 1

@KevinKevin

你的加入不是好的做法。请注意“除以零错误”，我已添加。

以下是“普通”SQL中的示例：

  SELECT
    Marks.Student_ID AS Year,
    (
      case when COUNT(case when Marks.Year_Taken = 1 then Marks.Student_ID else 0 end) <> 0 
      then SUM(
        case when Marks.Year_Taken = 1 then Final_Mark else 0 end
      ) / COUNT(
        case when Marks.Year_Taken = 1 then Marks.Student_ID else 0 end
      ) else 0 end
    ) AS Year_1_Marks,
    (
      case when COUNT(case when Marks.Year_Taken = 3 then Marks.Student_ID else 0 end) <> 0 
      then SUM(
        case when Marks.Year_Taken = 3 then Final_Mark else 0 end
      ) / COUNT(
        case when Marks.Year_Taken = 3 then Marks.Student_ID else 0 end
      ) else 0 end
    ) AS Year_3_Marks
  FROM
    Marks
    INNER JOIN Students ON Students.Student_ID = Marks.Student_ID
  GROUP BY
    Marks.Student_ID

以下是“Access”SQL中的示例：

  SELECT
    Marks.Student_ID AS Year,
      Iif ( COUNT(Iif( Marks.Year_Taken = 1 ,Marks.Student_ID, 0))=0,0,
       SUM(Iif ( Marks.Year_Taken = 1,Final_Mark, 0) ) / COUNT(Iif( Marks.Year_Taken = 1 , Marks.Student_ID, 0)) 
    ) AS Year_1_Marks,
      Iif ( COUNT(Iif( Marks.Year_Taken = 3 , Marks.Student_ID, 0))=0,0,
       SUM(Iif ( Marks.Year_Taken = 3 Final_Mark, 0) ) / COUNT(Iif( Marks.Year_Taken = 3 , Marks.Student_ID, 0)) 
    ) AS Year_3_Marks,

  FROM
    Marks
    INNER JOIN Students ON Students.Student_ID = Marks.Student_ID
  GROUP BY
    Marks.Student_ID

Answer 2

你应该使用一个联接。我会对您的代码进行一些可维护性和可能性能的更改，但只是为了给您一个想法，这应该可行。

Select A.Student_ID, A.Year_1_Marks, B.Year_3_Marks
   From (
SELECT Marks.Student_ID AS Year, SUM(Final_Mark)/COUNT(Marks.Student_ID) AS Year_1_Marks
FROM Marks, Students
WHERE Marks.Year_Taken = 1 AND Students.Student_ID IN
(SELECT Marks.Student_ID
FROM Marks, Students
WHERE Students.Student_ID = Marks.Student_ID) 
GROUP BY Marks.Student_ID
) A
Join (
SELECT Marks.Student_ID AS Year, SUM(Final_Mark)/COUNT(Marks.Student_ID) AS Year_3_Marks
FROM Marks, Students
WHERE Marks.Year_Taken = 3 AND Students.Student_ID IN
(SELECT Marks.Student_ID
FROM Marks, Students
WHERE Students.Student_ID = Marks.Student_ID) 
GROUP BY Marks.Student_ID
) B On B.Student_Id = A.Student_Id

Answer 3

首先，纠正您的格式，使其更具可读性。

然后你可以尝试这个，我认为它应该工作正常：

 SELECT Students.Student_ID as Student_ID
      , SUM(marks1.Final_Mark)/COUNT(marks1.Student_ID) AS Year_1_Marks
      , SUM(marks3.Final_Mark)/COUNT(marks3.Student_ID) AS Year_3_Marks   
   FROM Students   LEFT JOIN Marks marks1
     ON marks1.Student_ID = Students.Student_ID
     AND marks1.Year_Taken = 1   LEFT JOIN Marks marks3
     ON marks3.Student_ID = Students.Student_ID
     AND marks3.Year_Taken = 3

这显示了所有学生的分数，所以如果你有没有分数的学生，我会添加一个WHERE子句来排除它们以避免任何问题等。