Database Reference
In-Depth Information
Entity type
:
STUDENT
Entities
:
Karolyn Stone, Samuel Wang, Tabitha Peters, and Monika
Gonzales
Entity type
:
COURSE
Entities
:
Database Design, Data Warehousing, Web Page Design
Associations
:
Karolyn Stone with Database Design
Karolyn Stone with Data Warehousing
Karolyn Stone with Web Page Design
Associations
:
Database Design with Karolyn Stone
Database Design with Samuel Wang
Database Design with Tabitha Peters
Database Design with Monika Gonzales
Note the above associations and express these in E-R model terminology. Care-
fully examine the two sets of associations. What is the observation? In the first set
of associations, an individual entity Karolyn Stone of entity type STUDENT asso-
ciates with other individual entities Database Design, Data Warehousing, and Web
Page Design of the other entity type, COURSE. If entities of one entity type asso-
ciate with entities of another entity type, you express these associations in a E-R
data model diagram by joining the rectangular boxes representing the two entity
types with a relationship line. Although the two entity types are linked together to
express the relationship, remember that the associations are actually between indi-
vidual entities of these two entity types.
Figure 7-11 indicates how the above relationship is represented in an E-R
diagram. Look at the associations between individual entities illustrated in the
figure. As you know from our study of object-based data modeling, this is a many-
to-many relationship. Observe the notation in the figure to denote a many-to-many
relationship. This notation has acquired the name “crow's feet.” Compare this nota-
tion with the notation used in object-based data model for a many-to-many rela-
tionship. Usage of a specific notation is just a matter of convention and acceptance.
In practice, you will come across different sets of notations in common usage.
Appendix C covers diagramming conventions and symbols.
STUDENT
COURSE
Karolyn Stone
Database Design
Samuel Wang
Tabitha Peters
Data Warehousing
Web Page Design
Monika Gonzales
Figure 7-11
Relationship between entity types.
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