An Entity Relationship Diagram (ERD) or ER diagram is the best visual representation of a Database. Any Database Designer will follow Entity Relationship Diagram to design physical database. An Entity-Relationship diagram (ERD) typically serves as the main deliverable of a conceptual data model. While newer approaches to E-R modeling have developed, the E-R approach is still cited by some professionals as "the premier model for conceptual database design". An ERD is a logical representation of an organization's data, and consists of three primary components:
Entities -- Major categories of data and are represented by rectangles
Attributes -- Characteristics of entities and are listed within entity rectangles
Relationships -- Business relationships between entities and are represented by lines
An Entity is a person, place, object, event, or concept that an organization wants to maintain data on. Each entity has a unique identity that differentiates it from other entities. A point of distinction must be made between entity types and entity instances. An entity type is a collection of entities that share common properties. Entity types are also known as entity classes. An entity instance is an individual occurrence of an entity type. A data model describes an entity type only once; however there may be numerous instances of that type within a database.
Lets take an example and solve it using ER diagrams :
Entities -- Major categories of data and are represented by rectangles
Attributes -- Characteristics of entities and are listed within entity rectangles
Relationships -- Business relationships between entities and are represented by lines
An Entity is a person, place, object, event, or concept that an organization wants to maintain data on. Each entity has a unique identity that differentiates it from other entities. A point of distinction must be made between entity types and entity instances. An entity type is a collection of entities that share common properties. Entity types are also known as entity classes. An entity instance is an individual occurrence of an entity type. A data model describes an entity type only once; however there may be numerous instances of that type within a database.
Lets take an example and solve it using ER diagrams :
This above ER diagram example describe this problem:
A publishing company produces books on various subjects. The books are written by authors who specialize in one particular subject. The company employs editors who, not necessarily being specialists in a particular area, each take sole responsibility for editing one or more book publications. Every book require some items for publication. These items supplied by suppliers. One supplier can supply many items. Shop owner buys books from the publisher. Shop owner can buy many books but one book can be bought by one shop owner only. Books are uniquely identified by Bookid.
