Building the Analysis Model › Refining the Model › Handling Unusual Situations › Entity Types With Too Few Relationships and Attributes

Entity Types With Too Few Relationships and Attributes

Each entity type must have at least one attribute and one relationship membership. This makes sense because an entity type with only one relationship membership and no attributes can convey no additional information. Therefore, certain combinations of relationships and attributes should be checked carefully:

• Entity types with no attributes
  • Evaluate these carefully to ensure that there really are values associated with them; otherwise, they should be removed from the model. Also ensure that the pairings in which they participate are truly of interest to the business.
  • An entity type with no attributes is commonly an associative entity type that resolves one or more M:N relationships. There is always the possibility of discovering some attributes later on, such as the time and reason for the association. Adding an entity type or a relationship has a much greater impact on design aspects of the model such as Action Diagrams, the screen designs, and the Data Structure Diagram than adding an attribute to an entity type.
• Entity types with only one attribute
  • These are very rare, but occasionally they are valid as might be the case with a multi-valued attribute. Always check that the sole attribute does not belong to some other entity type.
• Isolated entity types
  • These have no relationships, and so the information provided by their attributes cannot be related to any other entity types in the business. This is an unlikely situation, so the definition is probably incorrect.
  • Most things of value to a business relate to some other aspects of the business. Early in building a model, important reference entities such as tax rates may appear to be isolated but are eventually associated with entity types such as state or product group.
  • If data design work has been done, you may find that some designer-added entity types appear in the model. Where such entity types appear, there may be some that are isolated, such as those that represent code tables. These isolated entity types may safely remain in the model. If, after careful consideration, you discover no relationships, you should remove the entity type from the model.