Primary Key vs Foreign Key in MySQL: Key Differences

Primary keys and foreign keys are the glue of relational databases. One identifies a row uniquely; the other creates a relationship between tables. Every JOIN you will ever write in analytics depends on these two concepts.

Key Points

• Primary key: unique identifier of a row in its own table (e.g., customers.id)
• Foreign key: a column that references a primary key in another table (e.g., orders.customer_id)
• A table has only one primary key but can have many foreign keys
• Primary keys reject NULLs; foreign keys may allow them
• Foreign keys can cascade updates/deletes with ON DELETE CASCADE
• Together they enable one-to-many and many-to-many relationships