SQL Server 复杂联接

Question

我有 3 个表格，如下所示

我编写了一个 SQL 脚本来返回特定学期和班级中所有学生科目的分数，无论分数是否被记录。但是下面的脚本只返回每个科目的记录分数。

select 
    a.StudentId, b.SubjectId, c.Score as tCA, d.Score as tExam 
from 
    tbl_StudentToClass a
join 
    tbl_SubjectToClass b on a.ClassId = b.ClassId
join 
    (select 
         sum(Score) AS Score, SubjectId, StudentId, TermId 
     from 
         tbl_Score 
     where 
         ScoreType = 1 
     group by 
         SubjectId, StudentId, TermId) c on b.SubjectId = c.SubjectId
     join 
    (select 
         sum(Score) AS Score, SubjectId, StudentId, TermId 
     from 
         tbl_Score 
     where 
         ScoreType = 2 
     group by 
         SubjectId, StudentId, TermId) d on b.SubjectId = d.SubjectId
where 
    a.ClassId = 2 and
    a.SchoolSessionId = 5 and 
    c.StudentId = a.StudentId and 
    c.TermId = 8 and
    d.StudentId = a.StudentId and 
    d.TermId = 8

如何返回所选班级的所有科目，无论是否有分数？

样本数据：

Answer 1

您正在加入表 C，如果不存在分数，则丢弃该行。

您应该在 C 中使用左连接，就像在 D 中使用的那样。

希望对你有帮助！

---

此外，我会将与左连接表相关的所有条件放在视图中，而不是放在 where 部分。

试试这个版本，告诉我它现在是否返回所有你想要的记录（抱歉，我现在没有激活的 SQL Server，我无法测试它）

select 
    a.StudentId, b.SubjectId, c.Score as tCA, d.Score as t 
from 
    tbl_StudentToClass a
join 
    tbl_SubjectToClassArm b on a.ClassToClassArmId = b.ClassToClassArmId
left join 
    (select 
         sum(Score) AS Score, SubjectId, StudentId, TermId 
     from 
         tbl_Score 
     where 
         ScoreType = 1 and TermId = 1100
     group by 
         SubjectId, StudentId, TermId) c on b.SubjectId = c.SubjectId
left join 
    (select 
         sum(Score) AS Score, SubjectId, StudentId, TermId 
     from 
         tbl_Score 
     where 
         ScoreType = 2 and TermId = 1100
     group by 
         SubjectId, StudentId, TermId) d on b.SubjectId = d.SubjectId
where 
    a.ClassToClassArmId = 135 and
    a.SchoolSessionId = 1069 and 
    c.StudentId = a.StudentId and 
    d.StudentId = a.StudentId