Database Reference
In-Depth Information
Table 2.7
Symbols for Relationships
Symbol
Description
Child entity with zero or more children
Zero or one child
Z
One or more children
P
Ranging from
n
to
m
children
n-m
Exactly one child
1
Optional parent
Identifying relationship
Nonidentifying relationship
key attributes are not null, the courses that do not have a prerequisite will not be present in the
intersection entity. For the meaning of the letter
P
, refer to Table 2.7.
Ternary relationship
: here are three entities involved in a ternary relationship. For example,
from the requirement analysis, one or more instances of the FACULTY entity are associated with
one or more instances of the CLASS entity and one or more instances of the CLASSROOM
entity. his relationship is a many-to-many-to-many ternary relationship shown in Figure 2.13.
In some cases, a ternary relationship can be treated as two separate binary relationships. In
this case, consider using binary relationships irst for simplicity. It is possible to break a ternary
relationship into binary relationships. In the above example, we can treat the ternary relationship
as two binary relationships shown in Figure 2.14.
here are some other types of ternary relationships. Let us consider the following examples
that are not included in our requirement analysis. Suppose that each course can only be taught
C_F_C
FACULT Y
CLASSROOM
ClassroomID
BuildingID
ClassID (FK)
BuildingID (FK)
ClassroomID (FK)
FacultyID (FK)
FacultyID
FirstName
LastName
Department
Capacity
CLASS
ClassID
Time
Semester
CourseName
Figure 2.13
Many-to-many-to-many ternary relationship.
