Relationship Participation

1
Relationship Participation

• Optional
• One entity occurrence (row) does not require a
  corresponding occurence (row) in related entity
• Shown by drawing a small circle on side of
  optional entity on ERD
• The existence of optionality indicates that the
  minimum cardinality is 0 for the optional entity.
• Mandatory
• One entity occurrence (row) requires a
  corresponding occurrence (row) in related entity
• If no optionality symbol is shown on ERD, it is
  mandatory
• The existence of mandatory relationship indicates
  that the minimum cardinality is 1 for the
  mandatory entity

2
Relationship Participation (Cont.)
An Optional CLASS Entity in the Relationship
PROFESSOR teaches CLASS (If PROFESSOR conducts
research without teaching CLASSes).
CLASS is optional to the PROFESSOR PROFESSOR is
mandatory to the CLASS
3
Relationship Participation (Cont.)
CLASS is optional to COURSE
4
Relationship Participation (Cont.)
COURSE and CLASS in a mandatory relationship
5
Relationship Degree

• Indicates number of associated entities
• Unary
• When an association maintained within a
• single entity
• Recursive
• Exists between occurrences of same entity set
• Binary
• When two entities are associated
• Ternary
• When three entities are
• associated

Unary
6
Relationship Degree (Cont.)

• Unary Relationship
• Recursive Relationship

Implementation of 1M EMPLOYEE manages EMPLOYEE
recursive relationship.
An employee within the EMPLOYEE entity manages
another EMPLOYEE within that entity. An EMPLOYEE
manages an EMPLOYEE. EMPLOYEE has a relationship
with itself.
7
Relationship Degree (Cont.)

• Ternary Relationship

A contributor can make donations to many funds, a
fund can have many contributors. A fund may
support many researchers, who become the funds
recipients, and a researcher may be supported by
many funds.
8
Relationship Degree (Cont.)
The Implementation of a Ternary Relationship
9
Composite Entities (Bridge Entities)

• Used to bridge between MN relationships
• Bridge entities composed of primary keys of
  entities to be connected

A CLASS may exist (at least at the start of
registration) even though it contains no
registration at all. STUDENT is optional to the
CLASS. To be classified as a STUDENT, a person
must be enrolled in at least one CLASS.
CLASS is mandatory to STUDENT.
10
Composite Entities (Cont.)
Converting the MN Relationship into Two 1M
Relationships
11
Composite Entities (Cont.)
A Composite Entity in an ERD