Attribute domain

In computing, the attribute domain is the set of values allowed in an attribute.^[1]

For example:

Rooms in hotel (1–300)
Age (1–99)
Married (yes or no)
Nationality (Nepalese, Indian, American, or British)
Colors (Red, Yellow, Green)

For the relational model it is a requirement that each part of a tuple be atomic.^[2] The consequence is that each value in the tuple must be of some basic type, like a string or an integer. For the elementary type to be atomic it cannot be broken into more pieces. Alas, the domain is an elementary type, and attribute domain the domain a given attribute belongs to an abstraction belonging to or characteristic of an entity.^{[clarification needed]}

For example, in SQL, one can create their own domain for an attribute with the command

CREATE DOMAIN SSN_TYPE AS CHAR(9);

The above command says: "Create a datatype SSN_TYPE that is of character type with size 9".