根据条件声明变量

Question

我想在下面声明一个变量@Grade：

Declare @Grade varchar(max)''
Select @Grade = ...

连接表需要满足以下条件：

如果 table1 subjectid 存在于 tableA id 中，则为每个 studentid 从 tableA 中选择 @Grade

如果 table1 subjectid 存在于 tableB id 中，则为每个 studentid 从 tableB 中选择 @Grade

如果tableC id中存在table1 subjectid，则为每个studentid从tableC中选择@Grade

Sample Table1 - Student
Student ID   Subject ID   Rating
100          200          A
101          200          B
102          300          A
103          400          B
104
105          300          A

Sample TableA - Chinese
Subject ID  Rating      Grade
200         A           Good
200         B           Poor

Sample TableB - English
Subject ID  Rating       Grade
300         A            Good
300         B            Poor

Sample TableC - Maths
Subject ID  Rating       Grade
400         A            Good
400         B            Poor

Expected output:
Student ID   Subject ID   @Grade
100          200          Good
101          200          Poor
102          300          Good
103          400          Poor
104
105          300          Good

有人可以帮我解决这个问题吗？

Answer 1

这似乎产生了你预期的结果：

declare @Student table (StudentID int not null,SubjectID int null,Rating char(1) null)
insert into @Student(StudentID,SubjectID,Rating) values
(100,200 ,'A'),    (101,200 ,'B'),    (102,300 ,'A'),
(103,400 ,'B'),    (104,null,null),    (105,300 ,'A')

declare @Chinese table (SubjectID int not null,Rating char(1) not null,Grade varchar(17) not null)
insert into @Chinese(SubjectID,Rating,Grade) values
(200,'A','Good'),    (200,'B','Poor')

declare @English table (SubjectID int not null,Rating char(1) not null,Grade varchar(17) not null)
insert into @English(SubjectID,Rating,Grade) values
(300,'A','Good'),    (300,'B','Poor')

declare @Maths table (SubjectID int not null,Rating char(1) not null,Grade varchar(17) not null)
insert into @Maths(SubjectID,Rating,Grade) values
(400,'A','Good'),    (400,'B','Poor')

select
    s.StudentID,
    s.SubjectID,
    COALESCE(c.Grade,e.Grade,m.Grade) as Grade
from
    @Student s
        left join
    @Chinese c
        on
            s.SubjectID = c.SubjectID and s.Rating = c.Rating
        left join
    @English e
        on
            s.SubjectID = e.SubjectID and s.Rating = e.Rating
        left join
    @Maths m
        on
            s.SubjectID = m.SubjectID and s.Rating = m.Rating

结果：

StudentID   SubjectID   Grade
----------- ----------- -----------------
100         200         Good
101         200         Poor
102         300         Good
103         400         Poor
104         NULL        NULL
105         300         Good

请注意，尽管您当前的表格设计在这里对我们没有帮助 - 我不清楚为什么我们有三个单独的表格用于主题标记方案 - 所有数据看起来都可以出现在一个表格中，这将大大简化上面的查询。