'there is no unique constraint matching given keys for referenced table' where i dont want something to be unique - postgresql

Question

In Postgres I have written the following code:
CREATE TABLE students (
student_id BIGINT PRIMARY KEY,
first_name VARCHAR(15),
surname VARCHAR(35),
enrollment_year INT);
CREATE TABLE teachers (
bsn BIGINT PRIMARY KEY,
first_name VARCHAR(15),
surname VARCHAR(35),
salary REAL,
scale INT,
CONSTRAINT salary CHECK(salary < (25 * scale) AND salary > (20 * scale))
);
CREATE TABLE course (
course_code VARCHAR(10) PRIMARY KEY,
course_name VARCHAR(20) UNIQUE,
study_points INT);
CREATE TABLE study_program (
course_name VARCHAR(20) PRIMARY KEY,
level VARCHAR(15),
duration VARCHAR(10),
FOREIGN KEY(course_name) REFERENCES course(course_name));
CREATE TABLE assignment (
assignment_code VARCHAR(10) UNIQUE,
course_name VARCHAR(20),
PRIMARY KEY(assignment_code, course_name),
FOREIGN KEY(course_name) REFERENCES course(course_name)
);
CREATE TABLE records (
student_id BIGINT,
course_name VARCHAR(20),
PRIMARY KEY(student_id, course_name),
FOREIGN KEY(student_id) REFERENCES students(student_id),
FOREIGN KEY(course_name) REFERENCES course(course_name));
CREATE TABLE make (
student_id BIGINT,
assignment_code VARCHAR(20),
PRIMARY KEY(student_id, assignment_code),
FOREIGN KEY(student_id) REFERENCES students(student_id),
FOREIGN KEY(assignment_code) REFERENCES assignment(assignment_code));
CREATE TABLE prerequisit (
assignment_code VARCHAR(20),
course_name VARCHAR(20),
PRIMARY KEY(assignment_code, course_name),
FOREIGN KEY(assignment_code) REFERENCES assignment(assignment_code),
FOREIGN KEY(course_name) REFERENCES course(course_name)
);
CREATE TABLE records_2 (
assignment_code VARCHAR(20),
course_name VARCHAR(20),
bsn BIGINT,
mandatory BOOLEAN,
year INT,
week INT,
PRIMARY KEY(assignment_code, course_name),
FOREIGN KEY(assignment_code) REFERENCES assignment(assignment_code),
FOREIGN KEY(course_name) REFERENCES course(course_name)
);
CREATE TABLE designes (
course_code VARCHAR(15),
bsn BIGINT,
study_points INT,
course_name VARCHAR(20),
PRIMARY KEY(course_code, bsn),
FOREIGN KEY(course_code) REFERENCES course(course_code),
FOREIGN KEY(bsn) REFERENCES teachers(bsn),
FOREIGN KEY(study_points) REFERENCES course(study_points),
FOREIGN KEY(course_name) REFERENCES course(course_name)
);
CREATE TABLE reviews (
bsn BIGINT PRIMARY KEY,
course_code VARCHAR(15),
study_points INT,
course_name VARCHAR(20),
FOREIGN KEY(bsn) REFERENCES teachers(bsn),
FOREIGN KEY(course_code) REFERENCES course(course_code),
FOREIGN KEY(study_points) REFERENCES course(study_points),
FOREIGN KEY(course_name) REFERENCES course(course_name)
);
The error I get is
ERROR: there is no unique constraint matching given keys for referenced table "course"
This error is fixed by adding the UNIQUE constraint to study points, however I do not want study points to be unique because I need different courses to have the same amount of study points.

Answer 1

Referencing study_points makes no sense whatsoever. In fact referencing three different columns of a single table makes no sense whatsoever.
You only need FOREIGN KEY(course_code) REFERENCES course(course_code) and you don't need the columns course_name and study_points in the reviews table as they are implicitly defined through the foreign key to the course table.
Which means the table should look something like this:
CREATE TABLE reviews
(
bsn BIGINT PRIMARY KEY,
course_code VARCHAR(15),
FOREIGN KEY(bsn) REFERENCES teachers(bsn),
FOREIGN KEY(course_code) REFERENCES course(course_code)
);
Which then begs the question: what is that table supposed to do? You can only have a single row per teacher in there because bsn is the primary key (thus can't be repeated). So that is a one-to-one (or one-to-zero/one) relationship. Which in turn means you could move the column course_code to the teachers table, e.g. as reviewed_course.
However, I guess you meant to create a many-to-many relationship between teachers and course which then should be something like this:
CREATE TABLE reviews
(
bsn BIGINT,
course_code VARCHAR(15),
primary key (bsn, course_code),
FOREIGN KEY(bsn) REFERENCES teachers(bsn),
FOREIGN KEY(course_code) REFERENCES course(course_code)
);
That makes the combination of bsn and course_code unique. Which means each teacher can review each course exactly once, but a teacher can review multiple courses and a single course can be reviewed by multiple teachers.