Schema design organizes data into tables and defines relationships between them. Understanding relationship types is fundamental to building robust databases.

Four relationship types:

1. One-to-One (1:1): User ↔ UserProfile

FK in either table with UNIQUE constraint

2. One-to-Many (1:N): Customer → Orders

FK in "many" side (orders.customer_id)

3. Many-to-Many (M:N): Students ↔ Courses

Junction table with two FKs

4. Self-Referencing: Employee → Manager

FK references same table's PK

CASCADE options:

• ON DELETE CASCADE: Delete parent → delete children

• ON DELETE RESTRICT: Prevent deletion if children exist

• ON DELETE SET NULL: Delete parent → set FK to NULL

Schema design is fundamental to database architecture. Poor design leads to data inconsistencies, complex queries, and scalability issues.

Every database has relationships: customers→orders (1:N), students↔courses (M:N), employees→manager (self-referencing). Proper design prevents data inconsistencies.