INFORMATION TECHNOLOGY EDUCATION DEPARTMENT
ITED123
(DATABASE MANAGEMENT SYSTEMS 1)
EXERCISE
3
ENHANCED ENTITY-RELATIONSHIP DIAGRAM
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I. OBJECTIVES
At the end of this exercise, students must be able to:
a) Identify the hierarchical relationships of entities based on a set of information
requirements
b) Design an EERD in Oracle SQL Developer Data Modeler
II. BACKGROUND INFORMATION
Enhanced Entity-Relationship Model
The Enhanced Entity-Relationship model is the extension of the original ER model with
new modeling constructs. The new modeling constructs introduced in the EER model are
supertype/subtype relationships.
Enhanced ER model = ER model + hierarchical relationships
Supertype and Subtype
Supertype is a generic entity type that has a relationship with one or more subtypes.
A subtype is a subgrouping of the entities in an entity type that is meaningful to the
organization. Subtypes inherit the attributes and relationships associated with their supertype.
Consider the entity type STUDENT (supertype), which has two subtypes
UNDERGRADUATE and POSTGRADUATE.
Generalization and Specialization
Generalization is the process of minimizing the differences between entities by
identifying common features. It can also be defined as the process of defining a generalized
entity type from a set of entity types.
Specialization is the process of identifying subsets of an entity set (the superset) that
share some distinguishing characteristics. In specialization the supertype is defined first and
the subtypes are defined next.
Figure 3.1 Generalization and Specialization
Constraints on Specialization and Generalization
1. Disjointness constraint allows us to specify whether an instance of a supertype may
simultaneously be a member of two or more subtypes.
a. Overlap refers to the fact that the same entity instance may be a member of
more than one subtype of the specialization.
b. Disjoint refers to the fact that the same entity instance may be a member of
only one subtype of the specialization.
2. Completeness constraint addresses the question whether an instance of a supertype
must also be a member of at least one subtype.
a. Total completeness refers to the fact that every entity instance in the supertype
must be a member of some subtype in the specialization.
b. Patial completeness refers to the fact that an entity instance in the supertype
need not be a member of any subtype in the specialization.
III. EXPERIMENTAL PROCEDURE
Overview
Consider an online auction database system in which members (buyers and sellers)
participate in the sale of items. The data requirements for this system are summarized as
follows:
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The online site has members who are identified by a unique member id and are
described by an email address, their name, a password, their home address, and a
phone number.
A member may be a buyer or a seller. A buyer has a shipping address recorded in the
database. A seller has a bank account number and routing number recorded in the
database.
Items are placed by a seller for sale and are identified by a unique item number
assigned by the system. Items are also described by an item title, an item description,
a starting bid price, bidding increment, the start date of the auction, and the end date
of the auction.
Items are also categorized based on a fixed classification hierarchy (for example a
modem may be classified as /COMPUTER/HARDWARE/MODEM).
Buyers make bids for items they are interested in. A bidding price and time of bid
placement is recorded. The person at the end of the auction with the highest bid price
is declared the winner and a transaction between the buyer and the seller may proceed
soon after.
Buyers and sellers may place feedback ratings on the purchase or sale of an item. The
feedback contains a rating between 1 and 10 and a comment. Note that the ratings are
placed on a completed transaction by the buyer or seller of the item in the transaction.
Task 1
Construct an enhanced entity-relationship diagram for the auction database.
Task 2
Draw an enhanced entity-relationship diagram using the Oracle SQL Developer Data
Modeler based from your answer in Task 1.
IV. QUESTION AND ANSWER
1. State two conditions that indicate when a designer should consider using
superype/subtype relationships.
a.
b.
There are attributes that apply to some (but not all) of the instances of an entity
type.
The instances of a subtype participate in the relationship unique to that subtype.
2. What is attribute inheritance? Why is it important?
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V. REFERENCES
Sumathi, S. (2007). Fundamentals of Relational Database Management Systems, Springer.