Introduction to database management systems

The Conceptual view of a database is represented by ER model. It allows to define a representation of real world concepts at logical level.ER model has no facilities to describe machine-related aspects. 

In ER model logical structure of data is captured by indicating grouping of data

Into entities. The ER model also supports a top-down approach by which detail can be

given in successive stages.

Entity

An entity is something which is described in a database by storing its data, it may be a concrete entity a conceptual entity. It is real-world object or concept (e.g., student, employee, course, department, or event)

Attributes

An attribute describes a property associated with entities. Attribute will have a

Name and a Value for each entity. (e.g., age, height, color). Some of Attribute types are: 

• Simple attribute − they are atomic values, which cannot be divided further. For example, a Customer phone number is an atomic value of 10 digits. 

• Composite attribute − Composite attributes are made of more than one simple attribute. For example, a Customer complete name will have first_name and last_name. 

• Derived attribute − Derived attributes are the ones that do not exist in physical databases, but their values are derived from other attributes present in database. For example, a Customer age can be derived from data_of_birth. 

• Single-value attribute − Single-value attributes contain single value. For example, Social_Security_Number. 

• Multi-value attribute − Multi-value attributes may contain more than one values. For example, a Customer can multiple phone number, email_address, etc.

Entity-Set and Keys

Key is a collection of attributes that uniquely identifies an entity among entity set.

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

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

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

Relationship

A relationship relates two or more distinct entities with a specific meaning.

For example, EMPLOYEE Smith works on ProductX PROJECT 

Relationships of the same type are grouped or typed into a relationship type.

For example, the WORKS_ON relationship defines a relation to PROJECTs on which EMPLOYEEs work on.

Relationship Set

The relationship is core aspect of a relational database. The different type of Relationship are:

• One to One: One entity from entity set A can be associated with at most one entity of entity set B and vice versa.
  
  
  
  
• One-to-many : One entity from entity set A can be associated with more than one entities of entity set B however an entity from entity set B, can be associated with at most one entity.
  
  
  
  
• Many to One : More than one entities from entity set A can be associated with at most one entity of entity set B, however an entity from entity set B can be associated with more than one entity from entity set A.