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Analysis and Design
Degree of an Entity Relationship Type
Relationships exhibit certain characteristics like degree, connectivity, and
cardinality. Once the relationships are identified their degree and cardinality
are also specified.
Degree: The degree of a relationship is the number of entities
associated with the relationship. The n-ary relationship is the general form for
degree n. Special cases are the binary, and ternary, where the degree is 2, and
Binary relationships, the association between two entities are the most common
type in the real world.
Fig 7.11 shows a binary relationship between member and book entities of library
Fig. 7.11 Binary Relationship
A ternary relationship involves three entities and is used when a binary relationship
is inadequate. Many modeling approaches recognize only binary relationships. Ternary
or n-ary relationships are decomposed into two or more binary relationships.
Connectivity and Cardinality
By connectivity we mean how many instances of one entity are associated with
how many instances of other entity in a relationship. Cardinality is used to specify
such connectivity. The connectivity of a relationship describes the mapping of
associated entity instances in the relationship. The values of connectivity are
"one" or "many". The cardinality of a relationship is the
actual number of related occurrences for each of the two entities. The basic types
of connectivity for relations are: one-to-one, one-to-many, and many-tomany.
A one-to-one (1:1) relationship is when at most one instance
of an entity A is associated with one instance of entity B. For example, take
the relationship between board members and offices, where each office is held
by one member and no member may hold more than one office.
A one-to-many (1:N) relationship is when for one instance
of entity A, there are zero, one, or many instances of entity B but for one instance
of entity B, there is only one instance of entity A. An example of a 1:N relationships
a department has many employees;
each employee is assigned to one department.
A many-to-many (M:N) relationship, sometimes called non-specific,
is when for one instance of entity A, there are zero, one, or many instances of
entity B and for one instance of entity B there are zero, one, or many instances
of entity A. An example is employees may be assigned to no more than three projects
at a time; every project has at least two employees assigned to it.
Here the cardinality of the relationship from employees to projects is three;
from projects to employees, the cardinality is two. Therefore, this relationship
can be classified as a many-to-many relationship.
If a relationship can have a cardinality of zero, it is an optional relationship.
If it must have a cardinality of at least one, the relationship is mandatory.
Optional relationships are typically indicated by the conditional tense. For example,
An employee may be assigned to a project.
Mandatory relationships, on the other hand, are indicated by words such as
must have. For example,
a student must register for at least three courses in each semester.
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