Many-to-Many Relationships
A many-to-many relationship occurs when multiple records in one table are associated with multiple records in another table. For example, consider a school database where students can enroll in multiple courses, and each course can have multiple students. This relationship cannot be represented directly within two tables without redundancy and inefficiency.
To manage many-to-many relationships, a third table, known as a join table (or junction table), is used. This table breaks down the many-to-many relationship into two one-to-many relationships. The join table contains foreign keys referencing the primary keys of the two tables involved in the relationship.
Consider the following tables:
- Students
- student_id
- student_name
- Courses
- course_id
- course_name
To represent the many-to-many relationship, we can introduce a join table:
- Enrollments
- student_id
- course_id
Using this schema:
- Each record in the Enrollments table links a student to a course.
- A student can have multiple entries in the Enrollments table for different courses.
- A course can have multiple entries in the Enrollments table for different students.
Student Table Course Table
Enrollment Table