First Normal Form

First normal form (1NF) is now considered to be part of the formal definition of a relation in the basic (flat) relational model. It states that:
1. the domain of an attribute must include only atomic (simple, indivisible) values and
2. that the value of any attribute in a tuple must be a single value from the domain of that attribute.
Hence, 1NF disallows having a set of values, a tuple of values, or a combination of both as an attribute value for a single tuple. In other words, 1NF disallows relations within relations or relationsas attribute values within tuples.
The only attribute values permitted by 1NF are single atomic (or indivisible) values.



Consider the DEPARTMENT relation schema, whose primary key is Dnumber, and suppose that we extend it by including the Dlocations attribute.
We assume that each department can have a number of locations.
As we can see, this is not in 1NF because Dlocations is not an atomic attribute. There are two ways we look at the Dlocations attribute:
The domain of Dlocations contains atomic values, but some tuples can have a set of these values. In this case, Dlocations is not functionally dependent on the primary key Dnum.


First normal form also disallows multi-valued attributes that are themselves composite. These are called nested relations because each tuple can have a relation within it.
This procedure can be applied recursively to a relation with multiple-level nesting to unnest the relation into a set of 1NF relations. This is useful in converting an unnormalized relation schema with many levels of nesting into 1NF relations.