Cardinality

How many rows match? This determines whether your result grows, shrinks, or stays the same size. Cardinality describes how many rows in one table match rows in another. Understanding cardinality helps you predict what your join results will look like. One-to-One (1:1) Each row in table A matches exactly one row in table B, and vice versa. Example: users and user_profiles tables Result: Same number of rows as each input table. One-to-Many (1:N) Each row in table A can match multiple rows in table B. This is the most common relationship. Example: One customer has many orders Result: More rows than the "one" side table. Alice appears 3 times, Bob 2 times, Carol once. Many-to-Many (N:N) Multiple rows in table A can match multiple rows in table B. This often requires a "bridge" table. Example: