Relational Database Concept
A relational database is a collection of relations or two-dimensional tables. It stores all its data inside tables, and nothing more. All operations on data are done on the tables themselves or produces another tables as the result.

Components of the Relational Models

• Collection of objects or relations
• Set of operators to act on the relations
• Data integrity for accuracy and consistency

A table is a set of rows and columns. Each row is a set of columns with only one value for each. All rows from the same table have the same set of columns, although some columns may have NULL values, i.e. the values for that rows was not initialized. Note that a NULL value for a string column is different from an empty string. You should think about a NULL value as an "unknown" value. The rows from a relational table is analogous to a record, and the columns to a field.

For example, a company might want to store their employees information in a database. In a relational database, you create several table to store different pieces of information about your employees.

EMP Table
	+-------------------+---------------+----------------------------------+
	| EMPNO             |Name           | TITLE           | DEPTNO         |
	+===================+===============+==================================+
	| 100               |JOHN DOE       | MANAGER         | 10             |
	+-------------------+---------------+----------------------------------+
	| 101               |MARY JANE      | SALES           | 20             +
	+-------------------+---------------+----------------------------------+
	

DEPT Table
	+-------------------+-------------------+-----------------+
	| DEPTNO            | DEPT_Name         | LOCATION        |
	+===================+===================+=================+
	| 10                | SYSTEM            | NY              |
	+-------------------+-------------------+-----------------+
	| 20                | MARKETING         | Sales           |
	+-------------------+-------------------+-----------------+

Data Models
Models are a cornstone of design. The objective is to produce a model that fits a multitude of these uses, can be understood by an end user, and contains sufficient detail for a developer to build a database system.

Purpose of Models
Models help communicate the concepts in people's minds. They can be used to do the following:

• Communicate
• Categorize
• Describe
• Specify
• Investigate
• Evolve
• Analyze
• Imitate

Entity Relationship (ER) Modeling
In an effective system, data is divided into discrete categories or entities. An entity relationship (ER) model is an illustration of various entities in a business and the relationship between them. An ER model is derived from business specification or narratives and built during the analysis phase of the system development life cycle. ER models separate the information required by a business from the activities performed within a business. Although businesses can change their activities, the type of information trends to remain constant. Therefore, the data structures alos tend to be constant.