Entity - Relationship Model

The entity-relationship (ER) data model allows us to describe the data involved in a real-world enterprise in terms of objects and their relationships and is widely used to develop an initial database design. The ER model is important primarily for its role in database design. It provides useful concepts that allow us to move from an informal description of what users want from their database to a more detailed, and precise, description that can be implemented in a DBMS.

ENTITIES, ATTRIBUTES, AND ENTITY SETS
- A database can be modeled as:
o a collection of entities,
o relationship among entities.
- An entity is an object that exists and is distinguishable from other objects.
o Example: specific person, company, event, plant
- Entities have attributes.
o Example: people have names and addresses
- An entity set is a set of entities of the same type that share the same properties.
o Example: set of all persons, companies, trees, holidays.



- Attributes : An entity is represented by a set of attributes, that is descriptive properties possessed by all members of an entity set.
o E.g. Employee = (Name, Address, Age, Salary)



Types of Attributes
- SIMPLE attributes are attributes that are drawn from the atomic value domains.
E.g. Name = {John} ; Age = {23}
- COMPOSITE attributes: Attributes that consist of a hierarchy of attributes.
E.g. Address may consists of “Number”, “Street” and “Suburb” → Address = {59 + ‘Meek Street’ + ‘Kingsford’}
- SINGLE VALUED attributes: Attributes that have only one value for each entity.
E.g. Name, Age for EMPLOYEE
- MULTIVALUED attributes: Attributes that have a set of values for each entity.
E.g. Degrees of a person: ‘ BSc’ , ‘MIT’, ‘PhD’
- DERIVED attributes: Attributes Contain values that are calculated from other attributes.
E.g. Age can be derived from attribute DateOfBirth. In this situation, DateOfBirth might be called Stored Attribute.

- DOMAIN/VALUE SETS : Each simple attribute of an entity type is associated with a value set which specifies the set of values that may be assigned to that attribute for each individual entity.
- KEYS : An important constraint on the entities is the key. An entity type usually has an attribute whose values are distinct for each individual entity in the collection. Such an attribute is called a key attribute.
For example, for the entity SET EMPLOYEE = {EID, Name, Address, Age, Salary}
EID attribute is the unique key for entity set EMPLOYEE as no two employees can have two same EID. Some entity types have more than one key attribute. An entity type may also have no key.