First normal form

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. The first definition of the term, in a 1971 conference paper by Edgar Codd, defined a relation to be in first normal form when none of its domains have any sets as elements.

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.

First normal form enforces these criteria:

The following scenario illustrates how a database design might violate first normal form.

Below is a table that stores the names and telephone numbers of customers. One requirement though is to retain multiple telephone numbers for some customers. The simplest way of satisfying this requirement is to allow the "Telephone Number" column in any given row to contain more than one value:

Note that the telephone number column simply contains text: numbers of different formats, and more importantly, more than one number for two of the customers. We are duplicating related information in the same column. If we would be satisfied with such arbitrary text, we would be fine. But it's not arbitrary text at all: we obviously intended this column to contain telephone number(s). Seen as telephone numbers, the text is not atomic: it can be subdivided. As well, when seen as telephone numbers, the text contains more than one number in two of our rows. This representation of telephone numbers is not in first normal form: our columns contain non-atomic values, and they contain more than one of them.

To bring the model into the first normal form, we split the strings we used to hold our telephone number information into "atomic" (i.e. indivisible) entities: single phone numbers. And we ensure no row contains more than one phone number.

Note that the "ID" is no longer unique in this solution with duplicated customers. To uniquely identify a row, we need to use a combination of (ID, Telephone Number).

Another design for the same data makes use of two tables: a Customer Name table and a Customer Telephone Number table.

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