Tables

All data in relational database is stored in tables. Every tables has a table name and a set of columns and rows in which the data is stored. each column is given a column name, a data type(defining characteristics of the data to be entered in the column). Usually in a relational database, some of the columns in different tables contain the same information. In this way, the tables can refer to one another.

Views (virtual Tables) in SQL

Views (virtual Tables) in SQL: The SQL DDL includes commands for defining views.A views is customized presentation of the data from one or more tables. views derive their data from the tables on which they are based, which are known as base ables of the view.

Non procedural Language :SQL is a non procedural language because it:

·         Processess sets of records rather than just one at a time

·         provides automatics navigation to the data

·        system administrators

·         Database administrators

·         Securiuty administrators

·         Application programmers

 

SQL Data Manipulation Language(DML)

SQL (Structured Query Language) is a syntax for executing queries . But the SQL language also includes a syntax to update,insert and delete records.

These query and update commands together  form the Data Manipulation Language (DML) part of SQL:

·SELECT: extracts data from a database table

·UPDATE: updates data in a database  table

·DELETE :deletes data from a database table

Transaction Control Commands:

Transaction Control Commands manage changes made by Data Manipulation Language commands. These commands are listed in the following table.

Command	Purpose
COMMIT	To make permanent the changed made by statements issued at the beginning of transaction
ROLLBACK	To undo all changes since the beginning of a transaction or since a savepoint.
SAVEPOINT	To establish a point back to which you ma y roll.
SET TRANSACTION	To establish properties for the current transaction.

SQL Data Definition Language (DDL)

The Data Definition Language (DDL) part of SQL permits database tables to be created or deleted. We can also define indexes (keys), specify links between tables, and impose constraints between database tables.

The most important DDL statements in SQL are:

·         CREATE TABLE: create a new database table

·         ALTER TABLE: alters(change ) a database table

·         DROP TABLE: delete a database table

·         CREATE  INDEX: create an index(search key)

·         DROP INDEX: delete an intex

FEATURES OF SQL

SQL is the set of commands that programs and users may use to access data within the database that supports it.  Application programs and database tools often allow users to access the database without directly using SQL, however, these applications also translate the actions into SQL under the hood.

SQL has its root in the paper, “A Relational model of data for Large Shared Data Banks” ,published by DR E. F. Codd in june 1970 in the association of computer machinery (ACM) journal, Communications of the ACM. incidentally , the paper is also contained Codd’s model, which is now accepted as the definitive model for relational database management systems(RDBMS).

·         SQL stands for Structured Query Language

·         SQL allows you to access a data base

·         SQL is an ANSI standard computer language

·         SQL can execute queries against database

·         SQL can retrieve data from a database

·         SQL can insert new records in a database

·         SQL can delete records from a database

·         SQL can update records in a database

·         SQL is easy to learn

The SQL commands can be categorised in different groups for convenience. They are;

·         Basic Queries in SQL

·         More complex SQL Queries