SQL Server 2017

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CREATE TABLE Student
(
    student_id INT IDENTITY,
    name VARCHAR(20),
        CONSTRAINT PK_STUDENT_ID PRIMARY KEY (student_id)
)

CREATE TABLE Course
(
Course_id INT IDENTITY,
    NameCourse VARCHAR(20),
        CONSTRAINT PK_Course_ID PRIMARY KEY (Course_id)
)

CREATE TABLE CourseXStudent
(
    course_id INT  NOT NULL,
    stundent_id INT NOT NULL UNIQUE,

CONSTRAINT FK_COURSE_ID FOREIGN KEY (course_id)
    REFERENCES course(course_id),
		CONSTRAINT FK__STUDENT FOREIGN KEY (stundent_id)
    REFERENCES Student(student_id)
)

INSERT INTO Student VALUES('Edgar vazquez')
INSERT INTO Student VALUES('Daniel Romero')
INSERT INTO Student VALUES('Jose Velazquez')
INSERT INTO Course VALUES('DBA')
INSERT INTO Course VALUES('REDES')

INSERT INTO CourseXStudent VALUES(1,1)
INSERT INTO CourseXStudent VALUES(2,2)
INSERT INTO CourseXStudent VALUES(1,3)

SELECT S.name,C.NameCourse
FROM Student S
INNER JOIN CourseXStudent CS ON CS.stundent_id=S.student_id
INNER JOIN Course C ON C.Course_id= CS.course_id

--drop tABLE CourseXStudent
--DROP TABLE Course
--DROP TABLE Student

name	NameCourse
Edgar vazquez	DBA
Daniel Romero	REDES
Jose Velazquez	DBA

Msg 2627 Level 14 State 1 Line 3
Violation of UNIQUE KEY constraint 'UQ__CourseXS__3CF81D7DB29E86B7'. Cannot insert duplicate key in object 'dbo.CourseXStudent'. The duplicate key value is (1).
Msg 3621 Level 0 State 0 Line 3
The statement has been terminated.