Specialization Hierarchy
Entity super types and subtypes are organized in a specialization hierarchy, which depicts the arrangement of higher-level entity super types (parent entities) and lower-level entity subtypes (child entities).
The following Figure shows the specialization hierarchy formed by an EMPLOYEE super type and three entity subtypes—PILOT, MECHANIC, and ACCOUNTANT.
Specialization Hierarchy
The specialization hierarchy reflects the 1:1 relationship between EMPLOYEE and its subtypes.
For example, a PILOT subtype occurrence is related to one instance of the EMPLOYEE supertype, and a MECHANIC subtype occurrence is related to one instance of the EMPLOYEE supertype.
The relationships depicted within the specialization hierarchy are sometimes described in terms of “is-a” relationships.
For example, a pilot is an employee, a mechanic is an employee, and an accountant is an employee. It is important to understand that within a specialization hierarchy, a subtype can exist only within the context of a supertype, and every subtype can have only one supertype to which it is directly related.
However, a specialization hierarchy can have many levels of supertype/subtype relationships—that is, you can have a specialization hierarchy in which a supertype has many subtypes; in turn, one of the subtypes is the supertype to other lower-level subtypes.
Specialization hierarchies enable the data model to capture additional semantic content (meaning) into the ERD. A specialization hierarchy provides the means to:
• Support attribute inheritance.
• Define a special supertype attribute known as the subtype discriminator.
• Define disjoint/overlapping constraints and complete/partial constraints.
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