7. ENTITY RELATIONSHIP MODELLING
To complete the analysis for a database design it is necessary to determine what constitutes a valid value for an attribute. A domain for an attribute is its set of valid values which includes a choice of datatype, but a full specification of domain is typically more than that.
For instance, analysis for student identifiers may lead one to state that a student identifier is a positive whole number of exactly 7 digits with no leading zeros. The analysis of requirements for person names may lead one to state that the values stored in a database for a first name, last name, or middle name will not be more than 50 characters in length, and that names will not have any spaces at the beginning or end.
For each attribute one must determine its domain. More than one attribute can share the same domain. Knowing the underlying domains in your model is important. They help to complete your analysis, they are indispensable for coding programs, and they are useful for defining meaningful error messages.
Attribute domains are not usually shown in an ERD. Rather, domains are included in accompanying documentation which can be referred to when the database is being implemented.