E-R Diagram Theory

ER diagramming is a technique that is widely used in the world of business and information technology to show how information is stored within a business system.

ER diagram is a specialized graphic that defines the interrelationship between entities used in computing in regard to the organizaiton of data within database or information system. It is also called an entity relationship model. Is is an abstract and conceptual representation of data, and database modeling method, used to produce a type of conceptual schema or semantic data model of a system.

Data modeling is a technique aimed at optimizing the way that information is stored and used within an organization. It begins with the identification of the main data groups, for example the invoice, and continues by defining the detailed content of each of these groups. This results in structured definitions for all of the information that is stored and used within a given system. The technique provides a solide foundation for system design and a universal standard for system documentaiton.

Basic components of ER-Diagram are as follows:
Entity

An entity is a piece of data-an object or concept about which data is stored. It is an abstraction from the complexities of some domain. It can be applied to almost anything that is significant to the system being studied. For example, Customer, Loan, Student, Airport etc. It is represented by a box that contain the name of that entity. It may be a physical object such as a house or a car, an event such as a house sale or a car service, or a concept such as a customer transaction or order.

An entity-type is a category. An entity, is an instance of a given entity-type. There are usually many instances if an entity-type.
Attribute
It is a element of the data group of an entity. Each entity has a data group associated with it. For example it the Car is an entity, then Car Number, Car Model, Car color etc are the attributes of Car entity. Attributes are represented by oval or circles that contains the attribute name inside it. A given attribute belonging to a given entity occurrence can only have one value.

Relationship
It captures how two or more entities are related to one another. For example an owns relationship between a company and a computer. It is represented by diamond shape symbol.

Entity Keys

Each entity occurrence can be uniquely identified, by using an attribute or a combination of attributes that is called entity key.he primary key is the attribute (or group of attributes) that serve to uniquely identify each entity occurrence.

Solid lines are used to connect these symbols to make a complete ER diagram.
Relationship Cardinality

It is the degree or maximum cardinality of the relationship. There are many occurrences of the entity may relate to the other entity. There are three type of relationship between entities:

• One-to-one :One instance of an entity is associated with one other instance of another entity. For example, in a database of employees, each employee name is associated with only one social security number.
  
  
• One-to-many :One instance of an entity is associated with zero, one or many instances of another entity, but for one instance of other entity there is only one instance of entity one. For example, for a company with all employees working in one building, the building name is associated with many different employees, but those employees all share the same singular association with entity.
  
  
• Many-to-many :One instance of an entity is associated with one, zero or many instances of another entity and same as vise versa. For example, for a company in which all of its employees work on multiple projects, each instance of an employee is associated with many instances of a project, and at the same time, each instance of a project has multiple employees associated with it.
  
  

Following are the basic steps to start an ER diagram:

Define entites : these are usually nouns used in descriptions of the system, in the discussion of business rules, or in documentation;

Define Relationships: these are usually verbs used in descriptions of the system or in discussion of the business rules;

Add attributes to the relations: these are determined by the queries,and may also suggest new entities, e.g. grade; or they may suggest the need for keys or identifiers.

Add cardinality to the relations: Many-to-Many must be resolved to two one-to-manys with an additional entity