IMPORTANT CONCEPTS OF SQL

SQL

What is SQL ?
It is Structured Query Language

Data Base -
It is a collection of inter related Computerized information.

Data Base Management System

The software which is used to manage a data base is known as data base management system.
For example – SQL


Keys

1 – Primary Key

It is a key which is used to identify a particular record in any relation \ table
It insures no two values in any column can be same .

For example – Roll no , Admission no
i.e., As your roll no in your college or school or in university is unique and the roll number will identify you .

And as your admission number in your college or school or university is different from other admission numbers and your admission number will represent you.


2 – Candidate Key

A candidate key is a set of attributes ( columns ) which uniquely identify the record ( tuple / tuples ) in a relation .

There will be only one primary key in a relation but there may be more than one candidate key in any relation .


3 – Foreign key

The foreign which is used here is not the foreign / abroad you understand , this is different foreign which is used here .

Now come to concept –

 A Foreign key is a key which is used to link two tables together . a foreign key is a column or a communication of columns who’s value match a primary key in a different table .

Tuple

Tuple is a record in any relation or a complete world record is known as tuple .

Attribute

The fields of any table is known as attributes .

Domain

A complete one column with values is known as domain .

Entity

A name given to any relation to identify that particular relation is known as entity .