所以我有以下表格让我有点头疼。我想要做的是创建一个查询，返回给定课程的所有作业，其中包含作业信息（成绩，平均成绩等），如果学生没有提交或未交付作业，则只在这些字段中为空。我试过left join，但我无法弄清楚这些表格。它们虽然古老而且结构糟糕，但它只是我所拥有的。
CourseInst_Students 在课程实例中为学生注册
| ID_CourseInst| SSN |
| =============|=====================|
| 24744 | 080590-XXXX | my ssn
Assignments_CourseInst 注册课程实例的作业
| ID_Assignment| ID_CourseInst |
| =============|=====================|
| 37978 | 24744 |
| 37979 | 24744 |
| 37992 | 24744 |
| 38046 | 24744 |
分配
| ID_Assignment| Title |
| =============|===========================|
| 37978 | Og þá var kátt í höllinni |
| 37979 | Test for Assignments |
| 37992 | Test 2 |
| 38046 | Eitthvað gott verkefni |
Assignments_Solutions 为一组学生（1对多）保留信息（成绩，提交日期等）。如果学生没有提交他们的解决方案，就没有记录。
| ID_Assignment| ID_Group | Grade |
| =============|===========================|=========|
| 37978 | 808046 | 10 | only one group has handed in.
Assignments_Solutions_Groups 将学生的SSN注册到一个群组。
| ID_Group | SSN |
| =============|===========================|
| 808046 | 221180-XXXX | not my SSN (see mine above)
搜索我的SSN的作业时，我的首选结果如下：
| ID_Assignment| Title | Grade |
| =============|===========================|=========|
| 37978 | Og þá var kátt í höllinni | NULL |
| 37979 | Test for Assignments | NULL |
| 37992 | Test 2 | NULL |
| 38046 | Eitthvað gott verkefni | NULL |
有人可以解决这个问题吗？谢谢！
更新
这是我到目前为止所做的事情
select A.ID_Assignment, A.Title, ASo.Grade
from CourseInst_Students as CS
join Assignments_CourseInst as AC on CS.ID_CourseInst= AC.ID_CourseInst
join Assignments as A on AC.ID_Assignment = A.ID_Assignment
left join Assignments_Solutions as ASo on A.ID_Assignment = ASo.ID_Assignment
left join Assignments_Solutions_Groups as ASGs on ASo.ID_Group = ASGs.ID_Group
where CS.SSN = '080590-XXXX'
AND CS.id_namskeid = 24744
我得到以下结果：
| ID_Assignment| Title | Grade |
| =============|===========================|=========|
| 37978 | Og þá var kátt í höllinni | 10 | <- this is someone else's handin. I want NULL here
| 37979 | Test for Assignments | NULL |
| 37992 | Test 2 | NULL |
| 38046 | Eitthvað gott verkefni | NULL |
答案 0 :(得分：1)
假设它是Oracle，问题在于设计（绝对垃圾）。如果学生尚未注册，则学生无法提交。在您的情况下，学生1已注册但学生2已提交。您将最终使用笛卡尔积来获得所需的结果。询问设计这个的人，阅读SQL的基础知识。
WITH C_S
AS (SELECT
'24744' AS ID_COURSEINST,
'080590-XXXX' AS SSN
FROM
DUAL
UNION ALL
SELECT
'24744' AS ID_COURSEINST,
'221180-XXXX' AS SSN
FROM
DUAL),
A_C
AS (SELECT
'37978' AS ID_ASSIGNMENT,
'24744' AS ID_COURSEINST
FROM
DUAL
UNION ALL
SELECT
'37979' AS ID_ASSIGNMENT,
'24744' AS ID_COURSEINST
FROM
DUAL
UNION ALL
SELECT
'37992' AS ID_ASSIGNMENT,
'24744' AS ID_COURSEINST
FROM
DUAL
UNION ALL
SELECT
'38046' AS ID_ASSIGNMENT,
'24744' AS ID_COURSEINST
FROM
DUAL),
ASSI
AS (SELECT
'37978' AS ID_ASSIGNMENT,
'Og þá var kátt í höllinni' AS TITLE
FROM
DUAL
UNION ALL
SELECT
'37979' AS ID_ASSIGNMENT,
'Test for Assignments' AS TITLE
FROM
DUAL
UNION ALL
SELECT
'37992' AS ID_ASSIGNMENT,
'Test 2' AS TITLE
FROM
DUAL
UNION ALL
SELECT
'38046' AS ID_ASSIGNMENT,
'Eitthvað gott verkefni' AS TITLE
FROM
DUAL),
A_S
AS (SELECT
'37978' AS ID_ASSIGNMENT,
'808046' AS ID_GROUP,
'10' AS GRADE
FROM
DUAL),
A_S_G
AS (SELECT
'808046' AS ID_GROUP,
'221180-XXXX' AS SSN
FROM
DUAL),
GRP_SUBMIT
AS (SELECT
A_S.ID_ASSIGNMENT ID_ASSIGNMENT_AS,
A_S.ID_GROUP ID_GROUP_AS,
A_S.GRADE,
C_S.ID_COURSEINST COURSE_SUB,
C_S.SSN SUBMIT_SSN
FROM
C_S,
A_S,
A_S_G
WHERE
A_S_G.SSN = C_S.SSN
AND A_S_G.ID_GROUP = A_S.ID_GROUP),
GRP_ASSIGN
AS (SELECT
A_C.ID_ASSIGNMENT ID_ASSIGNMENT_AC,
A_C.ID_COURSEINST AS ID_COURSEINST_AC,
ASSI.ID_ASSIGNMENT ID_ASSIGNMENT_ASSI,
ASSI.TITLE TITLE_ASSI,
C_S.ID_COURSEINST COURSE_ASSI,
C_S.SSN ASSI_SSN
FROM
A_C,
ASSI,
C_S
WHERE
A_C.ID_ASSIGNMENT = ASSI.ID_ASSIGNMENT)
SELECT
ID_ASSIGNMENT_ASSI,
TITLE_ASSI,
GRADE
FROM
(SELECT
*
FROM
GRP_SUBMIT
RIGHT OUTER JOIN
GRP_ASSIGN
ON ID_ASSIGNMENT_AS = ID_ASSIGNMENT_AC
AND SUBMIT_SSN = ASSI_SSN)
WHERE
ASSI_SSN = '221180-XXXX'
AND COURSE_ASSI = 24744
ORDER BY
ID_ASSIGNMENT_ASSI;
在GRP_SUBMIT中，我会检查提交作业的学生人数。
在GRP_ASSIGN中，我操纵笛卡尔积的预期提交数量。
在最终选择中，我让GRP_SUBMIT在子查询中的GRP_ASSIGN上有一个右外连接，然后根据你的搜索进行过滤。
<强>结果：
输入：221180-XXXX
ID_ASSIGNMENT_ASSI TITLE_ASSI GRADE
------------------ ------------------------- -----
37978 Og þá var kátt í höllinni 10
37979 Test for Assignments
37992 Test 2
38046 Eitthvað gott verkefni
4 rows selected.
输入：080590-XXXX
ID_ASSIGNMENT_ASSI TITLE_ASSI GRADE
------------------ ------------------------- -----
37978 Og þá var kátt í höllinni
37979 Test for Assignments
37992 Test 2
38046 Eitthvað gott verkefni
4 rows selected.
答案 1 :(得分：0)
您能分享到目前为止您尝试过的查询吗？另外，你使用的是MSSQL，MySQL，......？
你尝试过这样的事吗？
select a.ID_Assignment, a.Title, asol.Grade from Assignments a
left join Assignments_Solutions asol
on asol.ID_Assigment = a.ID_Assignment
left join Assignments_Solutions_Groups asg
on asg.ID_Group = asol.ID_Group
where asg.SSN = 'yourSocialSecurityNumber';