To complete the data model that Building a Relational Data Model describes, you must define a domain for each column. The domain of a column describes the constraints and identifies the set of valid values that attributes (columns) can assume.
The purpose of a domain is to guard the semantic integrity of the data in the model; that is, to ensure that it reflects reality in a sensible way. The integrity of the data model is at risk if you can substitute a name for a telephone number or if you can enter a fraction where only integers are valid values.
To define a domain, specify the constraints that a data value must satisfy before it can be part of the domain. To specify a column domain, use the following constraints: