Third Normal Form (3NF)

Concepts covered: dmThirdNormalForm

3NF: No Transitive Dependencies 3NF is the most practically important normal form because it catches the most common data duplication pattern: a column that depends on another non-key column instead of the primary key. This applies to every table, not just those with composite keys. Consider an orders table where each order has a customer_id, customer_name, and customer_city. customer_name and customer_city depend on customer_id, not on order_id. They are transitively dependent: order_id determines customer_id, which determines customer_name. Alice's name and city are stored twice. If Alice moves to Portland, you must update every order row. Miss one and the data contradicts itself. The test for any non-key column: does this value depend directly on the primary key, or does it depend on an

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