我有两个具有以下架构的 MySql 表

Courses(CNO:CHAR(10),CName:VARCHAR(50), DeptId:CHAR(3)): Each course is given a
unique ID called CNO
Register(RNO:CHAR(10), CNO:CHAR(10), Year:INTEGER, Sem:INTEGER, Grade:CHAR)

现在我必须找到所有（RNO，年份，Sem）三元组，以便学生在学期中拥有卷号 RNO 按年份定义，Sem 已修读/注册了至少三门或更多不同课程，每门课程均由不同部门提供。

我的代码如下

With tmp AS(
    SELECT Register.RNO,Register.CNO,Register.Year,Register.Sem,courses.DeptId, ROW_NUMBER() 
    OVER (PARTITION BY DeptId) 
    AS rn 
    from Register 
    inner join courses 
    on courses.CNO=Register.CNO
)
select RNO, Year, Sem
from tmp
where rn=1
GROUP BY RNO,Year,Sem 
having COUNT(CNO)>=3;

（我使用 Row_Number 在其中一列上应用 Distinct 关键字） 但是，如果学生在两个不同学期注册了三门以上课程，并且输出为 1 行而不是两行，则此代码会失败。为什么会发生这种情况以及如何解决

这是示例数据和架构的 SQL 代码

CREATE TABLE courses (
    CNO CHAR(10),
    CName VARCHAR(50),
    DeptId CHAR(3),
    PRIMARY KEY (CNO)
);



INSERT INTO courses (CNO,CName,DeptId)
VALUES ("CS1","database","CSE");
INSERT INTO courses (CNO,CName,DeptId)
VALUES ("CS2","deep","CSE");
INSERT INTO courses (CNO,CName,DeptId)
VALUES ("CS3","prob","CSE");
INSERT INTO courses (CNO,CName,DeptId)
VALUES ("MBA4","consume","IME");
INSERT INTO courses (CNO,CName,DeptId)
VALUES ("MBA5","Model","IME");
INSERT INTO courses (CNO,CName,DeptId)
VALUES ("MBA7","consume","IME");
INSERT INTO courses (CNO,CName,DeptId)
VALUES ("MBA8","Model","IME");
INSERT INTO courses (CNO,CName,DeptId)
VALUES ("SOC6","Social","HSS");
INSERT INTO courses (CNO,CName,DeptId)
VALUES ("MBA10","consume","IME");
INSERT INTO courses (CNO,CName,DeptId)
VALUES ("MBA11","Model","IME");
INSERT INTO courses (CNO,CName,DeptId)
VALUES ("SOC12","Social","HSS");
INSERT INTO courses (CNO,CName,DeptId)
VALUES ("SOC9","Social","HSS");

CREATE TABLE Register (
    RNO CHAR(10),
    CNO CHAR(10), 
    Year INTEGER, 
    Sem INTEGER,
    Grade CHAR,
    FOREIGN KEY (CNO) REFERENCES courses(CNO)
);    

INSERT INTO Register (RNO,CNO,Year,Sem,Grade)
VALUES ("1","CS1",1,1,"B");
INSERT INTO Register (RNO,CNO,Year,Sem,Grade)
VALUES ("1","CS2",1,1,"A");
INSERT INTO Register (RNO,CNO,Year,Sem,Grade)
VALUES ("1","CS3",1,1,"B");
INSERT INTO Register (RNO,CNO,Year,Sem,Grade)
VALUES ("1","MBA4",1,1,"B");
INSERT INTO Register (RNO,CNO,Year,Sem,Grade)
VALUES ("1","MBA5",1,1,"A");
INSERT INTO Register (RNO,CNO,Year,Sem,Grade)
VALUES ("1","SOC6",1,1,"A");

INSERT INTO Register (RNO,CNO,Year,Sem,Grade)
VALUES ("1","MBA7",2,2,"B");
INSERT INTO Register (RNO,CNO,Year,Sem,Grade)
VALUES ("1","MBA8",2,2,"A");
INSERT INTO Register (RNO,CNO,Year,Sem,Grade)
VALUES ("1","SOC9",2,2,"A");

INSERT INTO Register (RNO,CNO,Year,Sem,Grade)
VALUES ("1","MBA10",3,3,"B");
INSERT INTO Register (RNO,CNO,Year,Sem,Grade)
VALUES ("1","MBA11",4,3,"A");
INSERT INTO Register (RNO,CNO,Year,Sem,Grade)
VALUES ("1","SOC12",5,3,"A");

这里是 DB Fiddle 的链接。