Question

我想知道是否可以将两个查询合并为一个？我有两个SQL查询。第一个输出第一年级学生的平均最终成绩。第二个输出为三年级学生的平均最终成绩。

我尝试使用UNION，但是不幸的是，它只是将所有内容都移到了一列。我正在通过MS Access使用SQL。

第一个查询：

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

我希望将输出分成3列，分别是Student_ID，Year_1_Marks，Year_3_Marks。我们非常感谢您的帮助。

Answer 1

您应该使用联接。我将对您的代码进行一些更改以实现可维护性和可能的性能，但仅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 2

首先，请先对格式进行更正，以使其更具可读性。

然后您可以尝试一下，我认为它应该可以正常工作：

 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

这将显示所有学生的成绩，因此，如果您的学生没有成绩，我会添加一个WHERE子句以将其排除在外，以避免出现任何此类问题。

Answer 3

@KevinKevin

您的加入不是好习惯。当心“添加零错误”。

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

是否可以将两个SQL（MS Access）查询合并为一个？

3 个答案: