4.5

Supertype: A higher-level entity that encompasses lower-level entities called subtypes.
Subtype: A subset of the supertype, characterized by unique attributes and relationships.
Entire Subtype: Represents a complete subset of the supertype.
Entity Types: Define the specific characteristics of a supertype or subtype.
Attributes:
- Optional attributes of supertype become required in subtype.
- Enhances database semantics.
- Results in compact tables with fewer null values.
Relationships:
- Subtypes highlight subsets of data with different attributes and relationships than the supertype.

Repeated Attributes:
- An attribute found in several subtypes can be consolidated into a single representation.
Entities:
- Similar entities are often created from base entities due to common attributes.
Optional Attributes:
- Entities that share common attributes and relationships, facilitating clear organization.

Entity with Optional Attributes:
- An entity having many optional attributes can suggest the creation of new supertype/subtype entities.
- This entity becomes a super entity.
- Retains all required attributes, while optional attributes become required for new subtypes.
Partition Attribute:
- Indicates which subtype entity is linked to each instance of the supertype.
- Each partition corresponds to a distinct subtype association.

Instance Membership:
- Each entity instance can belong to multiple subtypes across different partitions.
Partition Attribute Values:
- Each value corresponds to a specific subtype relationship.
Weak Entities:
- All subtype entities are categorized as weak entities, but not all weak entities are subtype entities.

Creating Supertype/Subtype Entities:
- This action is the final step in a four-part conceptual design process:
  1. Discover entities, their relationships, and attributes.
  2. Determine cardinality between entities.
  3. Distinguish between strong and weak entities.
  4. Create the supertype/subtype structure.