Question

You are creating an enhanced entity relationship model for a campus information system. A junior team member who is helping you asks you to describe the following terms with examples of relevant entities from the system, for example, students, activities etc.

1.       Supertype and Subtype

2.       Disjoint and Overlapping Discriminator

3.       Total and Partial Completeness

4.       Attribute Inheritance

Answer 1

i) Super Type and Sub type:

A supertype is a generic entity type that has a relationship with one or more subtypes.A subtype is a sub-grouping of the entities in an entity type that is meaningful to the organization and that shares common attributes or relationships distinct from other subgroups.

• Subtypes inherit all supertype attributes

• Subtypes have attributes that are different from other subtype

ii) Disjoint and Overlapping Dicriminator:

In a disjoint constraint you would have to put the musician in either one or the other sub classes. In an overlapping constraint the musician can be put in both. The disjoint rule states an entity instance of a supertype can only be a member of one subtype.

iii)Total and Partial Completeness:

• Total Participation − Each entity is involved in the relationship. Total participation is represented by double lines.

• Partial participation − Not all entities are involved in the relationship. Partial participation is represented by single lines.

iv) Attribute Inheritance:

Attribute inheritance is a property that inherits all attribute values and all relationship instances of the super type to its entities of subtype.