Question

我有三张桌子

students_first_semester_mark (StudentdID,Subject, Semester_I_Mark )
students_second_semester_mark (StudentdID,Subject, Semester_II_Mark )
students_third_semester_mark (StudentdID,Subject, Semester_III_Mark )

我希望得到以下类型的输出：

_________________________________________________________________________________
|StudentdID  | Subject |  Semester_I_Mark|  Semester_II_Mark |  Semester_III_Mark
_________________________________________________________________________________

请注意，Semester_I_Mark或Semester_II_Mark可能包含空值。

我渴望得到你的答案。

Answer 1

这是你想要的吗？

SELECT [students_first_semester_mark].[StudentdID]
      ,[students_first_semester_mark].[Subject]
      ,[students_first_semester_mark].[Semester_I_Mark]
      ,[students_second_semester_mark].[Semester_II_Mark]
      ,[students_third_semester_mark].[Semester_III_Mark]
FROM [students_first_semester_mark]
JOIN [students_second_semester_mark] ON [students_second_semester_mark].[StudentdID] = [students_first_semester_mark].[StudentdID]
JOIN [students_third_semester_mark] ON [students_third_semester_mark].[StudentdID] = [students_second_semester_mark].[StudentdID]

Answer 2

我假设所有学生都不会出现在所有科目中。

在MySQL中实现它会有点棘手，因为它不支持full outer join。

要导出FULL OUTER JOIN，您需要LEFT JOIN UNION RIGHT JOIN。在你的情况下，有3个表，你需要重复前两个表，然后UNION它们。因此，查询将非常复杂。

DBFiddle Demo

select * from (
    select coalesce(fs.studentdid,t.studentdid) as STUDENTDID
    ,coalesce(fs.subject,t.subject) as subject 
    ,fs.semester_i_mark,fs.semester_ii_mark,t.semester_iii_mark
    from
    (
    select
    coalesce(f.studentdid,s.studentdid) as STUDENTDID,
    coalesce(f.subject,s.subject) as subject
    ,f.semester_i_mark,s.semester_ii_mark
    from
        STUDENTS_FIRST_SEMESTER_MARK f
        left join
        STUDENTS_SECOND_SEMESTER_MARK s
            on f.studentdid=s.studentdid
            and f.subject=s.subject
    UNION
    select
    coalesce(f.studentdid,s.studentdid) as STUDENTDID,
    coalesce(f.subject,s.subject) as subject
    ,f.semester_i_mark,s.semester_ii_mark
    from
        STUDENTS_FIRST_SEMESTER_MARK f
        right join
        STUDENTS_SECOND_SEMESTER_MARK s
            on f.studentdid=s.studentdid
            and f.subject=s.subject
    ) fs

    LEFT JOIN 
        STUDENTS_THIRD_SEMESTER_MARK t
            on fs.STUDENTDID=t.studentdid
            and fs.subject=t.subject
) u1
UNION
select * from (
    select coalesce(fs.studentdid,t.studentdid) as STUDENTDID
    ,coalesce(fs.subject,t.subject) as subject 
    ,fs.semester_i_mark,fs.semester_ii_mark,t.semester_iii_mark
    from
    (
    select
    coalesce(f.studentdid,s.studentdid) as STUDENTDID,
    coalesce(f.subject,s.subject) as subject
    ,f.semester_i_mark,s.semester_ii_mark
    from
        STUDENTS_FIRST_SEMESTER_MARK f
        left join
        STUDENTS_SECOND_SEMESTER_MARK s
            on f.studentdid=s.studentdid
            and f.subject=s.subject
    UNION
    select
    coalesce(f.studentdid,s.studentdid) as STUDENTDID,
    coalesce(f.subject,s.subject) as subject
    ,f.semester_i_mark,s.semester_ii_mark
    from
        STUDENTS_FIRST_SEMESTER_MARK f
        right join
        STUDENTS_SECOND_SEMESTER_MARK s
            on f.studentdid=s.studentdid
            and f.subject=s.subject
    ) fs

    RIGHT JOIN 
        STUDENTS_THIRD_SEMESTER_MARK t
            on fs.STUDENTDID=t.studentdid
            and fs.subject=t.subject
) u2

使用Coalesce，以便在我们使用studentdid时返回subject和outer join的任何非空值。

Answer 3

以下是我能想到的最简单的答案。

f5

Answer 4

这可能会对您有所帮助：

 SELECT table1.StudentdID, table1.Subject, 
  table1.Semester_I_Mark, table2.Semester_II_Mark, 
  table3.Semester_III_Mark 
 FROM students_first_semester_mark  table1
 INNER JOIN students_second_semester_mark  table2 
  ON table1.StudentdID = table2.StudentdID
 INNER JOIN students_third_semester_mark   table3 
  ON table1.StudentdID = table3.StudentdID

从具有相同ID的三个表中进行选择

4 个答案: