ER Modeling

ER Diagram

ER diagram or Entity Relationship diagram is a conceptual model that gives the graphical representation of the logical structure of the database.

• It shows all the constraints and relationships that exist among the different components.
• An ER diagram is mainly composed of following three components- Entity Sets, Attributes and Relationship Set.
• Roll_no is a primary key that can identify each entity uniquely.

Entity Set

An entity set is a set of the same type of entities.

• Strong Entity Set: A strong entity set is an entity set that contains sufficient attributes to uniquely identify all its entities. In other words, a primary key exists for a strong entity set. Primary key of a strong entity set is represented by underlining it.
• Weak Entity Set: A weak entity set is an entity set that does not contain sufficient attributes to uniquely identify its entities. In other words, a primary key does not exist for a weak entity set. However, it contains a partial key called a discriminator. Discriminator can identify a group of entities from the entity set. Discriminator is represented by underlining with a dashed line.

Relationship Sets

A relationship is defined as an association among several entities.

• Unary Relationship Set - Unary relationship set is a relationship set where only one entity set participates in a relationship set.
• Binary Relationship Set - Binary relationship set is a relationship set where two entity sets participate in a relationship set.
• Ternary Relationship Set - Ternary relationship set is a relationship set where three entity sets participate in a relationship set.
• N-ary Relationship Set - N-ary relationship set is a relationship set where ‘n’ entity sets participate in a relationship set.

Cardinality Constraint

Cardinality constraint defines the maximum number of relationship instances in which an entity can participate.

• One-to-One Cardinality - An entity in set A can be associated with at most one entity in set B. An entity in set B can be associated with at most one entity in set A.
• One-to-Many Cardinality - An entity in set A can be associated with any number (zero or more) of entities in set B. An entity in set B can be associated with at most one entity in set A.
• Many-to-One Cardinality - An entity in set A can be associated with at most one entity in set B. An entity in set B can be associated with any number of entities in set A.
• Many-to-Many Cardinality - An entity in set A can be associated with any number (zero or more) of entities in set B. An entity in set B can be associated with any number (zero or more) of entities in set A.

Types of Attributes

Attributes are the descriptive properties which are owned by each entity of an Entity Set.

• Simple Attributes - Simple attributes are those attributes which cannot be divided further. Ex. Age
• Composite Attributes - Composite attributes are those attributes which are composed of many other simple attributes. Ex. Name, Address
• Multi Valued Attributes - Multi valued attributes are those attributes which can take more than one value for a given entity from an entity set. Ex. Mobile No, Email ID
• Derived Attributes - Derived attributes are those attributes which can be derived from other attribute(s). Ex. Age can be derived from DOB.
• Key Attributes - Key attributes are those attributes which can identify an entity uniquely in an entity set. Ex. Roll No.