Cardinality Explained

Concepts: dmCardinalityRequired, dmOneToOne, dmOneToMany, dmManyToMany

Cardinality answers the question: for one row in table A, how many rows in table B can it relate to? Always read cardinality from both directions. The relationship between customers and orders is 1:N from the customer side, but 1:1 from the order side (each order belongs to exactly one customer). One-to-One (1:1) Each row in A maps to exactly one row in B. In practice, this usually means the two tables could be merged into one, but are kept separate for performance, security, or organizational reasons. A users table and a user_profiles table: every user has exactly one profile. Joining them never changes the row count. One-to-Many (1:N) One-to-many is the most common relationship in data modeling. One customer has many orders. One department has many employees. One product has many reviews