Mumbai University > Information Technology > Sem 3 > Database Management System

Marks: 5 M

Year: May 2015

1. Introduction:

i. The participation constraint specifies if an entity’s existence depends on its being related to any other entity via the relationship type.

ii. This constraint specifies the minimum number of relationship instances that each entity can participate in, and is sometimes called the minimum cardinality constraint.

2. Total Participation:

i. The participation of an entity set E in a relationship set R is said to be total if every entity in E participates in at least one relationship in R.

ii. Consider two entities EMPLOYEE and DEPARTMENT related with the help of relationship WORKS_FOR.

iii. If a company policy states that every employee must work for a department, then an employee entity can exist only if it participates in at least one WORKS_FOR relationship instance. Thus, the participation of EMPLOYEE in WORKS_FOR is called total participation, meaning that every entity in the total set of employee entities must be related to a department entity via WORKS_FOR.

iv. Total participation is also called existence dependency.

v. Total participation is displayed as a double line in ER diagram connecting the participating entity type to the relationship

3. Partial Participation:

i. If only some entities in entity set E participates in relationships in R, the participation of entity set E in relationship set R is said to be partial.

ii. Consider two entities EMPLOYEE and DEPARTMENT related with the help of relationship MANAGES.

iii. Here, not all employees are expected to manage a department, so the participation of EMPLOYEE in the MANAGES relationship type is partial.

iv. Thismeans that some or part of the set of employee entities are related to some department entity via MANAGES, but not necessarily all.