What is an Entity Relationship Diagram (ERD)?

Entity relationship diagrams  are a way to represent the structure and layout of a database. It is used frequently to describe the database schema. ER diagrams are very useful as the provide a good conceptual view of any database, regardless of the underlying hardware and software.

An ERD is a model that identifies the concepts or entities that exist in a system and the relationships between those entities. An ERD is often used as a way to visualize a relational database: each entity represents a database table, and the relationship lines represent the keys in one table that point to specific records in related tables. ERDs may also be more abstract, not necessarily capturing every table needed within a database, but serving to diagram the major concepts and relationships. This ERD is of the latter type, intended to present an abstract, theoretical view of the major entities and relationships needed for management of electronic resources. It may assist the database design process for an e-resource management system, but does not identify every table that would be necessary for an electronic resource management database.

Objects

There are three main objects on an ER Diagram: entities, relations and attributes. These are joined together with lines.

1.Entities:
An entity is a concept or object in the database. Entities are concepts within the data model. Each entity is represented by a box within the ERD. Entities are abstract concepts, each representing one or more instances of the concept in question. An entity might be considered a container that holds all of the instances of a particular thing in a system. Entities are equivalent to database tables in a relational database, with each row of the table representing an instance of that entity.

 

Each entity represents a container for instances of the thing in question. The diagram below has an entity for “student” and another for “school.” This indicates that the system being modeled may contain one or more students and one or more schools.

2.Relationships:-
Relations are the connections between two or more entities. Relationship lines indicate that each instance of an entity may have a relationship with instances of the connected entity, and vice versa.

The diagram above now indicates that  Employees may have some relationship with company.  More specifically, there may be a relationship between a particular Employee  and a particular company. Relations also have cardinality and degrees. The degree of a relationship is how many entities it connects to.

For example, there might be a relationship OWNS between entities PERSON and VEHICLE. Other types of relationships are: ISA relationships are for when one entity is also a particular type of another entity.
For example DOG <ISA> ANIMAL. All the attributes of the entity ANIMAL are automatically given to DOG.

                           
                                                            
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