ER Diagram

• 1976 proposed by Peter Chen

• An ER diagram is widely used in database design
• Represent the conceptual level of a database system
• Describe things and their relationships at a high level

Basic Concepts

• Entity set – an abstraction of similar things, 
• E.g:- cars, students
• An entity set contains many entities
• Entities are represented by means of rectangles. 
• Rectangles are named with the entity set they represent.
• E.g:- Teacher, Student, Project.

• Attributes: Common properties of the entities in an entity sets. Attributes are represented by means of ellipses. Every ellipse represents one attribute and is directly connected to its entity.

Attribute

• Composite VS Simple Attributes
• Single valued VS Multivalued Attributes
• Stored VS Derived Attributes
• Complex Attributes 

Relationship – specify the relations among entities from two or more entity sets

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 

One-to-many

many-to-one

many-to-many

Keys

Key is an attribute or collection of attributes that uniquely identifies an entity among the entity set.

1.    Super Key − A set of attributes (one or more) that collectively identifies an entity in an entity set.

2.    Candidate Key − A minimal super key is called a candidate key. An entity set may have more than one candidate key.

3.    Primary Key − A primary key is one of the candidate keys chosen by the database designer to uniquely identify the entity set.