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.
Cardinality of Relationship
While creating relationship between two entities, we may often need to face the cardinality problem. This simply means that how many entities of the first set are related to how many entities of the second set. Cardinality can be of the following three types.
One-to-One
Only one entity of the first set is related to only one entity of the second set. E.g. A teacher teaches a student. Only one teacher is teaching only one student. This can be expressed in the following diagram as:
One-to-Many
Only one entity of the first set is related to multiple entities of the second set. E.g. A teacher teaches students. Only one teacher is teaching many students. This can be expressed in the following diagram as:
Many-to-One
Multiple entities of the first set are related to multiple entities of the second set. E.g. Teachers teach a student. Many teachers are teaching only one student. This can be expressed in the following diagram as:
Many-to-Many
Multiple entities of the first set is related to multiple entities of the second set. E.g. Teachers teach students. In any school or college many teachers are teaching many students. This can be considered as a two way one-to-many relationship. This can be expressed in the following diagram as:
In this discussion we have not included the attributes, but you can understand that they can be used without any problem if we want to.
Participation Constraints
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.
Cardinality of Relationship
While creating relationship between two entities, we may often need to face the cardinality problem. This simply means that how many entities of the first set are related to how many entities of the second set. Cardinality can be of the following three types.
One-to-One
Only one entity of the first set is related to only one entity of the second set. E.g. A teacher teaches a student. Only one teacher is teaching only one student. This can be expressed in the following diagram as:
Only one entity of the first set is related to multiple entities of the second set. E.g. A teacher teaches students. Only one teacher is teaching many students. This can be expressed in the following diagram as:
Many-to-One
Multiple entities of the first set are related to multiple entities of the second set. E.g. Teachers teach a student. Many teachers are teaching only one student. This can be expressed in the following diagram as:
Many-to-Many
Multiple entities of the first set is related to multiple entities of the second set. E.g. Teachers teach students. In any school or college many teachers are teaching many students. This can be considered as a two way one-to-many relationship. This can be expressed in the following diagram as:
In this discussion we have not included the attributes, but you can understand that they can be used without any problem if we want to.
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.
No comments:
Post a Comment