[英]Retrieve only one row using left join based on foreign key
I have 2 relationship tables in the database.我在数据库中有 2 个关系表。
One is Class
一种是Class
create table Class(
CourseId int,
CourseName varchar(50),
LecturerId int,
Class1Time varchar(50),
Class2Time varchar(50),
Class1Place varchar(50),
Class2Place varchar(50)
);
and Lecturer
和Lecturer
create table Lecturer(
Id int,
FirstName varchar(50),
LastName varchar(50),
primary key(Id)
);
LecturerId
from Class
table is foreign key of Id
from Lecturer
. Class
表中的LecturerId
是Lecturer
中Id
的外键。 All I want is to select the data(one row) from both table based on the foreign key LecturerId
= specefic value
我想要的只是 select 基于外键LecturerId
= specefic value
的两个表中的数据(一行)
I've tried using left join like我试过使用左连接
select Class.CourseName, Class.CourseId, Class.Class1Time, Class.Class2Time, Class.Class1Place, Class.Class2Place, Lecturer.Id, Lecturer.FirstName, Lecturer.LastName
from Class
left join Lecturer
on Lecturer.Id = (select Lecturer.Id from Lecturer where LecturerId = Class.LecturerId)
where Class.LecturerId = someValue;
go
But it causes the error:但这会导致错误:
Subquery returned more than 1 value.子查询返回超过 1 个值。 This is not permitted when the subquery follows =, ,=, <, <=, >.当子查询跟随 =, ,=, <, <=, > 时,这是不允许的。 >= or when the subquery is used as an expression. >= 或当子查询用作表达式时。
For example in the Lecturer
table I have a Id = 2001
例如,在Lecturer
表中,我有一个Id = 2001
So I want perform the above query that need to return the below result like所以我想执行上面的查询,需要返回下面的结果,比如
CourseName - CourseId - Class1Time - Class2Time - Class1Place - Class2Place - Lecturer Infor(based on the Id)
Just only one row.只有一排。
Try this, using table aliases, with all columns aliased.试试这个,使用表别名,所有列都有别名。
select C.CourseName, C.CourseId, C.Class1Time, C.Class2Time, C.Class1Place, C.Class2Place, L.Id, L.FirstName, L.LastName
from Class C
left join Lecturer L on L.Id = C.LecturerId
where C.LecturerId = @Id;
The reason you get that error is because:您收到该错误的原因是:
(select Lecturer.Id from Lecturer where LecturerId = Class.LecturerId)
Is actually the same as:实际上是一样的:
(select Lecturer.Id from Lecturer where Class.LecturerId = Class.LecturerId)
Because Lecturer
doesn't have a column called LecturerId
, and because you haven't table qualified the column, SQL Server realises it does exist in Class
and therefore uses it.因为Lecturer
没有名为LecturerId
的列,并且因为您没有对该列进行表格限定,所以 SQL 服务器意识到它确实存在于Class
中,因此使用它。
And since Class.LecturerId = Class.LecturerId
is true for all records in the 'Lecturer' table you get all its ids returned.由于Class.LecturerId = Class.LecturerId
对于“Lecturer”表中的所有记录都为真,因此您将返回其所有 id。
And of course there is actually no reason to perform a subquery anyway, because you already have the value you are using the subquery to obtain.当然,实际上没有理由执行子查询,因为您已经拥有使用子查询获取的值。
Try the following please.请尝试以下方法。
DECLARE @LecturerId INT = 1
SELECT C.CourseName, C.CourseId, C.Class1Time, C.Class2Time, C.Class1Place, C.Class2Place, L.Id, L.FirstName, L.LastName
FROM Class AS C
INNER JOIN Lecturer AS L ON L.Id = C.LecturerId
INNER JOIN
(
SELECT C.CourseId, C.LecturerId
FROM Class AS C
WHERE C.LecturerId = @LecturerId
GROUP BY C.CourseId, C.LecturerId
) AS G ON G.CourseId = C.CourseId AND G.LecturerId = C.LecturerId
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