IS5413 Chap.5 Supertypes and Subtypes in the Enhanced ER Model
Supertypes and Subtypes in the Enhanced ER Model
One construct, added to the basic model
The enhanced model adds a general entity type with one or more specialised versions beneath it. The supertype holds the attributes and relationships shared by everything below it.
Each subtype adds only what is unique to it and inherits the rest, and the sentence that gives inheritance its meaning is that anything that is a member of a subtype is by that fact a member of the parent as well.
A subtype that ends up with no unique attributes and no unique relationships should not be drawn at all: in the published vehicle example the motorcycle loses its box once price, engine size and name have moved up to the supertype, and that is the correct outcome rather than an oversight.
Two constraints that answer different questions
Completeness asks a vertical question: does every member of the parent have to fall into some child?
Yes is total specialisation, drawn with a double line; no is partial specialisation, drawn with a single line. Disjointness asks a horizontal question: may one instance of the supertype belong to two subtypes at the same time? No is the disjoint rule, marked with a d; yes is the overlap rule, marked with an o.
The two are independent, so all four combinations are legal and a diagram carries both marks because neither implies the other.
This is the most common confusion in the chapter and the cure is simply to ask the vertical question and the horizontal question separately, in that order.
The discriminator takes its shape from the disjointness rule
The subtype discriminator sits on the parent and its value is what says which child a given member falls into, and its shape is decided rather than chosen.
Under the disjoint rule it is a simple attribute with one alternative value per subtype. Under the overlap rule it must be composite, with one part per subtype each holding a yes or no, because a single value cannot express two memberships at once.
In a hierarchy with more than one level, each level carries its own constraint pair and its own discriminator; treating the lower split as inheriting the upper one is a frequent error.
Two routes in, and where relationships attach
Generalisation works bottom up: several specific entity types are found to share attributes, so a more general parent is defined to hold them.
Specialisation works top down: one entity type turns out to have attributes applying only to some instances, so children are carved out. Both end with a supertype carrying the shared material. Where a relationship is drawn then asserts a business rule. A relationship at the supertype level says every subtype participates; one at a subtype level says only that subtype does.
The published hospital example puts the link to a responsible physician on the patient supertype because it applies to everyone, and the link to a bed on the resident subtype because only resident patients occupy one. Drawing that at the wrong level does not merely look untidy, it states something false.
What this chapter covers
- 01
Attribute inheritance and what it licenses
- 02
Generalisation against specialisation
- 03
Total against partial specialisation
- 04
The disjoint rule against the overlap rule
- 05
Discriminator shape under each rule
- 06
Relationships at supertype against subtype level
- 07
Packaged data models and what to check in them
Worked example · free
Fix both constraints and write the discriminator
- 2State the completeness rule with the words from the narrative that force it.
- 2State the disjointness rule with the words that force it.
- 2Write the discriminator and justify its shape.
- 2Say what the later requirement changes, and what it leaves alone.
Key terms
- Supertype
- A generic entity type holding the attributes and relationships shared by all of its subtypes, with its identifier inherited by each of them.
- Subtype
- A subgrouping of an entity type carrying attributes distinct from the other subgroupings. Membership of a subtype carries membership of the parent with it.
- Attribute Inheritance
- The property that a subtype instance carries the values of every supertype attribute, which is why shared attributes are never repeated in the subtypes.
- Total Specialisation
- The completeness rule under which no member of the parent may sit outside every child. Drawn as a double line.
- Disjoint Rule
- The constraint that an instance of the supertype may belong to only one subtype at a time. Marked with a d, and it forces a simple discriminator.
- Packaged Data Model
- A predefined, reusable model, either universal or industry specific, offered as a starting point whose embedded constraints must be checked against your own business.
Supertypes and Subtypes in the Enhanced ER Model FAQ
What is the difference between completeness and disjointness?
Completeness asks whether every parent instance reaches a child; disjointness asks whether one parent instance can reach two children. They are independent, so a hierarchy can be total and overlapping, total and disjoint, partial and overlapping, or partial and disjoint. Because neither implies the other, a correct diagram carries both marks, and a question giving you one of them is not giving you the other.
How do I know whether the discriminator is simple or composite?
The disjointness rule decides it, not preference. Under the disjoint rule a single valued attribute suffices, because each instance belongs to exactly one subtype and one value can name it. Under the overlap rule one value cannot express two simultaneous memberships, so the discriminator must be composite with one part per subtype, each holding a yes or no.
Should a subtype with no unique attributes still be drawn?
No. A subtype earns its box by carrying an attribute or a relationship the parent cannot, and an empty one adds a relation to the eventual schema for no benefit. The published generalisation example deliberately leaves one of its three original entity types without a subtype after the shared attributes move upward, and that absence is the correct answer rather than an omission.
Exam move
Write the four completeness and disjointness combinations as a two by two grid and put a real example in each cell from your own experience, not from the chapter. Then take any hierarchy with two levels and write both discriminators out in full, including every value or every yes and no part.
The discriminator is where the two constraints stop being abstract, so writing it is the fastest way to find out whether you decided them correctly.
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