What is an ER diagram?

An Entity-Relationship (ER) diagram expresses the logical structure of a database by showing the relationships between entity sets in a system. ER diagrams are widely used because they help structure information stored in a database and can be directly translated into tables.

Components

The following are the major components of an ER diagram:

• Entity

  An entity is a real-world object. It may or may not have a physical existence. Examples include person John and his bank account.

• Entity Set

  An entity set is a collection of similar entities. Examples include a group of people or a bank. An entity set is represented by a rectangle in an ER diagram.

• Attribute

  An attribute is the property of an entity. Examples include name, age, etc. An attribute is represented by an oval in an ER diagram.

• Relationship

  A relationship is an association between two or more entity sets. A relationship is represented by a diamond in an ER diagram.

• Cardinality

  Cardinality defines the attributes of a relationship using numbers. It includes:

  1. One-to-one relationship: An entity in entity set “a” is associated with, at most, one entity in entity set “B”.

  2. One-to-many relationship: An entity “a” is associated with more than one entity in “B”.

  3. Many-to-one relationship: Many entities in “a” are associated with an entity in “b”.

  4. Many-to-many relationship: Many entities in “a” are associated with many entities in “b”.

The following illustration shows an ER diagram: