Multiple sub-entities
Several sub-entities of the same entity:
• are not necessarily exclusive.
• do not necessarily partition the set.
Advantages of sub-entities
A sub-entity inherits all the attributes and associations of its super-entity, but can have attributes or associations that the super-entity does not have.
A sub-entity can also have specific attributes. These only have meaning for that particular sub-entity. In the above example:
• "Registry number" and "number of employees" only have meaning for a "company".
• "Date of birth" is a characteristic of a "person", not a "company".
A sub-entity can also have specific associations.
• A "person" falls into a "socio-professional group": “manager”, “employee”, “shopkeeper”, “grower”, etc. This classification makes no sense for a “company”. There is also a classification for companies, but this differs from the one for persons.