Relational Database Management Systems

A Relational Database Management system (often called an RDBMS for short) such as ORACLE gives you a way of doing the below mentioned tasks in an understandable and reasonably uncomplicated way :

Lets you put data into it
Keeps the data
Lets you get the data out and work with it
Provides us tools to capture, edit and manipulate data.
How to keep it securely
How to create and manipulate the data and create reports on it.

What is a Relational Model

The Relational model of a DBMS presents an orderly, predictable and intuitive approach to organizing, manipulating and viewing data. The relational model is based on mathematical concepts that on set theory, which makes it simple.

Definition and Terminology of RDBMS

The relational management system is a way of logically storing and managing data in the form of tables. A TABLE is a method of presenting data in the form of rows and columns.

Each column represents a single field of a record. Several such columns make a record, providing meaningful and interrelated information. Each record is represented in a row. A table may consist of any number of rows. Many tables having interrelated and interdependent data are grouped by establishing relations. Managing the tables and their relations provides mean's for inserting, deleting, retrieval and updating data forms an RDBMS.

Establishing Relations

Primary Key

We have seen that data is stored logically in tables in RDBMS. Each table has a unique name. To identify a particular row in a table, a column or a combination of columns are used. The column should be able to uniquely identify each row. Such a column is a candidate for a 'Primary Key'. No two rows in the table can have the same values for the column comprising the primary key. A column identified as primary key cannot have duplicate values and null values (no value). For example, considering the Employee table in fig 1.2, every employee has a unique employee number. The 'empno' column can be chosen as the primary key. Similarly the 'Deptno' column in the Department table can be the primary key.

Foreign Key

The Primary key and the Foreign key are used to establish relations among related tables. In fig 1.2 the domain of the values in the 'Dept No' column of the Employee table is within the range of values in the Department table's 'Dept No'. An employee must belong to a department that is listed in the Department table. The 'Dept No' column in table Employee is the candidate for the foreign key. By definition, a foreign key in a table shall be a primary key in some other related table.

Nulls

A Null can be said as an undefined value or no value. A null is used in the columns where the value is unknown. A null does not mean spaces or blanks. A 'null' value cannot be used in any calculations or comparison operations, A ',mill' is comparable to an infinity. A null is not. equal to another null.

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