For each of the following enterprise constraints decide whether - it is already

Question

For each of the following enterprise constraints decide whether

- it is already expressed in the Enhanced Entity-Relationship model (EER) in the diagram below

- it is not already expressed in the EER in the diagram below but could be expressed using only features of an EER model.

- cannot be expressed in the EER in the diagram below using only features of an EER model.

In each case explain your answer briefly but clearly.

(a) A table order originates from exactly one table of one restaurant

(b) Two table orders dispatched at (roughly) the same time must originate from different tables

Receives_by_phone Restaurant hone_line phone num Contains dateTi me billingNo total Order Originates from Table optional.orh number PhoneOrder TableOrder street wai ter deliveryAddress postcode city

Explanation / Answer

it is already expressed in the Enhanced Entity-Relationship model (EER) in the diagram below

- it is not already expressed in the EER in the diagram below but could be expressed using only features of an EER model.

- cannot be expressed in the EER in the diagram below using only features of an EER model.

In each case explain your answer briefly bu

One entity type might be a subtype of another--very similar to subclasses in OO programming

A relationship exists between a Freshman entity and the corresponding Student entity

This relationship is called IsA

Type Hierarchy

Associated constraints may apply:

Covering constraint: Union of subtype entities is equal to set of supertype entities. An entity is an element of at least one subtype

Disjointness constraint: Sets of subtype entities are disjoint from one another (i.e., the sets are mutually exclusive). An entity can be an element of at most one entity

Specialization Abstraction

Specialization-needed when an entity set has subsets that have special attributes or that participate in special relationships

Process of breaking up a class into subclasses

Ex: Faculty contains AdjunctFac and FullTimeFac

All Faculty have attributes facid, lastName, firstName, rank.

AdjunctFac also have coursePayRate

FullTimeFac have annualSalary

Union or Category

Subclass related to a collection of superclasses

Each instance of subclass belongs to one, not all, of the superclasses

Superclasses form a union or category

Ex. A Sponsor may be a team, a department, or a club –

Each Sponsor entity instance is a member of one of these superclasses, so Sponsor is a subclass of the union of Team, Dept, Club

EER diagram - connect each superclass to union circle, connect circle to subclass, with subset symbol on line bet circle and subclass

ory – every member of the sets that make up the union must participate

Shown on E-ER by double line from union circle to subset

See below every Concert or Fair must be a Campus-Wide Event

Partial category – not every member of the sets must participate

Shown by single line

See below-- not every Club, Team, or Dept must be a Sponsor

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