How to implement a many-to-many relationship?

A many-to-many relationship is implemented through an intermediate table (join table) in the database, which contains pairs of identifiers of related entities.

For example, there are entities Student and Course. One student can attend many courses, and one course can have many students.

Implementation in a relational database:

CREATE TABLE Student (
  id INT PRIMARY KEY,
  name VARCHAR(100)
);

CREATE TABLE Course (
  id INT PRIMARY KEY,
  title VARCHAR(100)
);

CREATE TABLE Student_Course (
  student_id INT,
  course_id INT,
  PRIMARY KEY (student_id, course_id),
  FOREIGN KEY (student_id) REFERENCES Student(id),
  FOREIGN KEY (course_id) REFERENCES Course(id)
);

In an object-oriented approach (for example, in ORM), this is reflected through collections with annotations or configurations indicating a many-to-many relationship.

Thus, the intermediate table links records of two tables, allowing the implementation of multiple relationships between them.