Final answer:
In the Extended Entity-Relationship Model, entity subtypes inherit attributes from their supertype and have a hierarchical relationship with it. Supertypes do not inherit from subtypes, and attributes can be shared between the two.
Step-by-step explanation:
When discussing the Extended Entity-Relationship Model (EERM), it's important to understand the concepts of entity subtypes and supertypes. Entity subtypes and supertypes provide a hierarchical way of organizing entities where subtypes represent a subset of the supertype entity that shares common features but also has distinctive features of its own.
To answer the question specifically:
- Entity subtypes inherit attributes from their supertype. This means that all attributes that are applicable to the supertype are also applicable to the subtype. For example, if 'Vehicle' is a supertype with attributes like 'make' and 'model', then a subtype 'Car' would inherit these attributes but could also have additional attributes specific to cars such as 'body style'.
- Entity supertypes do not inherit attributes from their subtypes. The supertype defines the common attributes that all its subtypes share.
- Subtypes and supertypes have a hierarchical relationship. The supertype stands at the top of the hierarchy, and the subtypes derive from it.
- Attributes are often shared between subtypes and supertypes as subtypes inherit common attributes from the supertype. However, subtypes can also have unique attributes that are not applicable to the supertype or other subtypes.
Understanding these relationships is critical when designing a database using the EERM, as it affects how tables are structured and how data integrity is maintained. In practice, this model allows for a more organized and efficient database design, reflecting the real-world entities more closely.