Database Reference
In-Depth Information
Let us highlight certain points about the ERD to indicate how it matches with
the stated information requirements.
For each course, there may exist one or more classes. Note the one-to-many
relationship between COURSE and CLASS.
The relationship between COURSE and CLASS is an identifying relationship.
CLASS depends on COURSE for its existence. CLASS is a weak entity
type.
Observe the one-to-many relationship between TEXTBOOK and COURSE.
Although a specific textbook is assigned to a course, the relationship allows the
possibility of the same textbook being used for more than one course.
Why is the attribute Author shown as a multivalued attribute? This is to provide
for multiple authors for the same textbook.
The entity type EXAM TYPE includes examinations such as Mid-Term,
Quizzes, Assignments, Laboratory Work, Final, and so on.
The entity type GRADE represents the three-way relationship among CLASS,
STUDENT, and EXAM TYPE.
Although faculty members teach specific courses, the teaching assignments are
for teaching particular classes of these courses. Note the direct relationship
between FACULTY and CLASS.
The attribute Score refers to the score for a given student, in a certain class,
and for a particular examination type.
CHAPTER SUMMARY
Like object-based modeling, entity-relationship modeling is another generic
data modeling technique. E-R data modeling seems to be a natural way of
looking at real-world information.
Many data modelers prefer E-R data modeling because several CASE tools
support it.
Components or building blocks of E-R data model: entity type, attribute,
primary key, relationship, cardinality indicators, generalization/specialization.
The E-R data model provides additional notations to indicate variations in the
components.
An entity type represents all similar entities; on the flip side, an entity is an
instance or occurrence of an entity type.
Attribute types: single-valued and multivalued, simple and composite, derived.
One of the candidate keys is chosen as the primary key for an entity type. A
composite key consists of more than one attribute.
Relationship between two entity types expresses the associations between
entities of one entity type and entities of the second entity type. The cardinality
indicators denote how many instances of one entity type may be associated
with one instance of the other entity type.
The minimum cardinality indicator represents optional or mandatory nature of
a relationship.
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