Entity Relationship Diagram
Let us now learn how the ER Model is represented by means of an ER diagram. Any
object, for example, entities, attributes of an entity, relationship sets, and attributes of
relationship sets, can be represented with the help of an ER diagram.
Entity
Entities are represented by means of rectangles. Rectangles are named with the entity
set they represent.
Attributes
Attributes are the properties of entities. Attributes are represented by means of ellipses.
Every ellipse represents one attribute and is directly connected to its entity (rectangle).
If the attributes are composite, they are further divided in a tree like structure. Every
node is then connected to its attribute. That is, composite attributes are represented by
ellipses that are connected with an ellipse.
Multivalued attributes are depicted by double ellipse.
Derived attributes are depicted by dashed ellipse.
Relationship
Relationships are represented by diamond-shaped box. Name of the relationship is
written inside the diamond-box. All the entities (rectangles) participating in a
relationship, are connected to it by a line.
Binary Relationship and Cardinality
A relationship where two entities are participating is called a binary relationship.
Cardinality is the number of instance of an entity from a relation that can be associated
with the relation.
∙ One-to-one − When only one instance of an entity is associated with the
relationship, it is marked as '1:1'. The following image reflects that only one
instance of each entity should be associated with the relationship. It depicts
one-to-one relationship.
∙
One-to-many − When more than one instance of an entity is associated with a
relationship, it is marked as '1:N'. The following image reflects that only one instance
of
entity on the left and more than one instance of an entity on the right can be
associated with the relationship. It depicts one-to-many relationship.
∙
Many-to-one − When more than one instance of entity is associated with the
relationship, it is marked as 'N:1'. The following image reflects that more than one
instance of an entity on the left and only one instance of an entity on the right
can be associated with the relationship. It depicts many-to-one relationship.
∙
Many-to-many − The following image reflects that more than one instance of an
entity on the left and more than one instance of an entity on the right can be
associated with the relationship. It depicts many-to-many relationship.
Participation Constraints
∙ Total Participation − Each entity is involved in the relationship. Total participation
is represented by double lines.
∙ Partial participation − Not all entities are involved in the relationship. Partial
participation is represented by single lines.
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