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First normal form enforces these criteria: One requirement though is to record multiple telephone numbers for some customers.The simplest way of satisfying this requirement is to allow the "Telephone Number" field in any given record to contain more than one value: Assuming, however, that the Telephone Number column is defined on some telephone number-like domain, such as the domain of 12-character strings, this representation is not in first normal form.In the first normal form, the previous table can be represented in the following way.

Indeed, repeating groups of telephone numbers do not occur in this design.

Instead, each Customer-to-Telephone Number link appears on its own record.

With Customer ID as key, a one-to-many relationship exists between the two tables.

A record in the "parent" table, Customer Name, can have many telephone number records in the "child" table, Customer Telephone Number, but each telephone number belongs to one, and only one customer.

First normal form (1NF) is a property of a relation in a relational database.

A relation is in first normal form if and only if the domain of each attribute contains only atomic (indivisible) values, and the value of each attribute contains only a single value from that domain.

First normal form is an essential property of a relation in a relational database.

Database normalization is the process of representing a database in terms of relations in standard normal forms, where first normal is a minimal requirement.

It is worth noting that this design meets the additional requirements for second and third normal form. Codd's definition of 1NF makes reference to the concept of 'atomicity'.

Codd states that the "values in the domains on which each relation is defined are required to be atomic with respect to the DBMS." meaning a field should not be divided into parts with more than one kind of data in it such that what one part means to the DBMS depends on another part of the same field.

Hugh Darwen and Chris Date have suggested that Codd's concept of an "atomic value" is ambiguous, and that this ambiguity has led to widespread confusion about how 1NF should be understood.