Domain key normal form

After a SQL database is in third normal form, you’ve eliminated most, but not all, chances of modification anomalies. Normal forms beyond the third are defined to squash those few remaining bugs.

Domain key normal form (DK/NF)

Boyce-Codd normal form (BCNF), fourth normal form (4NF), and fifth normal form (5NF) are examples of such forms. Each form eliminates a possible modification anomaly but doesn’t guarantee prevention of all possible modification anomalies. Domain-key normal form, however, provides such a guarantee.

A relation is in domain key normal form (DK/NF) if every constraint on the relation is a logical consequence of the definition of keys and domains. A constraint in this definition is any rule that’s precise enough that you can evaluate whether or not it’s true. A key is a unique identifier of a row in a table. A domain is the set of permitted values of an attribute.

Look at this database, which is in 1NF, to see what you must do to put that database in DK/NF.

domain-key normal form

Table: SALES (Customer_ID, Product, Price)

Key: Customer_ID


  • Customer_ID determines Product

  • Product determines Price

  • Customer_ID must be an integer > 1000

To enforce Constraint 3 (that Customer_ID must be an integer greater than 1000), you can simply define the domain for Customer_ID to incorporate this constraint. That makes the constraint a logical consequence of the domain of the CustomerID column. Product depends on Customer_ID, and Customer_ID is a key, so you have no problem with Constraint 1, which is a logical consequence of the definition of the key.

Constraint 2 is a problem. Price depends on (is a logical consequence of) Product, and Product isn’t a key. The solution is to divide the SALES table into two tables. One table uses Customer_ID as a key, and the other uses Product as a key. The database, besides being in 3NF, is also in DK/NF.

Design your databases so they’re in DK/NF if possible. If you can do that, enforcing key and domain restrictions causes all constraints to be met, and modification anomalies aren’t possible. If a database’s structure is designed to prevent you from putting it into DK/NF, then you have to build the constraints into the application program that uses the database. The database itself doesn’t guarantee that the constraints will be met.