Relationships, Relationship Sets and Relationship Types
In this lesson, we will study the basics of relationships between entities.
We'll cover the following
Relationships, relationship sets and relationship types#
A relationship is an association between two entities, for example, an entity EMPLOYEE WORKS_ON PROJECT, which is another entity. However, note that different instances of employees will be working on different projects. So an employee, John WORKS_ON Project Netlife and maybe some other employee Sara (who is also an instance of an employee) WORKS_ON Project mForm.
So we can say that relationship type is simply the relationship that exists between two entities like WORKS_ON. While the set of similar associations at a point of time is called the Relationship Set.
The illustration below will help clear up the confusion:
Similar to the case of entity types and entity sets, a relationship type and its corresponding relationship set are customarily referred to by the same name.
Representation in ER diagrams#
In ER diagrams, relationship types are displayed as diamond-shaped boxes, which are connected by straight lines to the rectangular boxes that represent the participating entity types. The relationship name is displayed in the diamond-shaped box. The diagram below illustrates this concept:
In the next lesson, we will learn about the degrees of relationship type.
