Junction Table

A junction table is a table used to represent a Many-to-Many relationship between two tables.

Since relational databases don’t support N:M relationships directly, you decompose them into two 1:N relationships using this intermediate table.

Structure

A junction table typically contains:

• Foreign Keys to both related tables
• Often a composite primary key made from those foreign keys

Example

Students ⇐> Course

Enrollments(
    student_id → Students.id,
    course_id → Courses.id,
    PRIMARY KEY (student_id, course_id)
)

Key Properties

• Each row represent one association
• Prevents duplicate relationships (via composite PK)
• Can store extra attributes about the relationship: grade, enrolled_at, status