Structured Query Language




SQL tutorial gives unique learning on Structured Query Language and it helps to make practice on SQL commands which provides immediate results.
SQL is a language of database, it includes database creation, deletion, fetching rows and modifying rows etc.
SQL is an ANSI (American National Standards Institute) standard but there are many different versions of the SQL language.
What is SQL?
SQL is Structured Query Language, which is a computer language for storing, manipulating and retrieving data stored in relational database. SQL is the standard language for Relation Database System. All relational database management systems like MySQL, MS Access, Oracle, Sybase, Informix, postgres and SQL Server use SQL as standard database language.
Also, they are using different dialects, such as:
• MS SQL Server using T-SQL,
• Oracle using PL/SQL,
• MS Access version of SQL is called JET SQL (native format) etc.
Why SQL?
• Allows users to access data in relational database management systems.
• Allows users to describe the data.
• Allows users to define the data in database and manipulate that data.
• Allows to embed within other languages using SQL modules, libraries & pre-compilers.
• Allows users to create and drop databases and tables.
• Allows users to create view, stored procedure, functions in a database.
• Allows users to set permissions on tables, procedures, and views.
When you are executing an SQL command for any RDBMS, the system determines the best way to carry out your request and SQL engine figures out how to interpret the task.
There are various components included in the process. These components are Query Dispatcher, Optimization Engines, Classic Query Engine and SQL Query Engine, etc.
Classic query engine handles all non-SQL queries but SQL query engine won't handle logical files. Following is a simple diagram showing SQL Architecture:

The standard SQL commands to interact with relational databases are CREATE, SELECT, INSERT, UPDATE, DELETE and DROP.
These commands can be classified into groups based on their nature :

DDL - Data Definition Language :

Command Description
CREATE Creates a new table, a view of a table, or other object in database
ALTER Modifies an existing database object, such as a table
DROP Deletes an entire table, a view of a table or other object in the database

DML - Data Manipulation Language :

Command Description
SELECT Retrieves certain records from one or more tables
INSERT Creates a record
UPDATE Modifies records
DELETE Deletes records

DCL - Data Control Language :

Command Description
GRANT Gives a privilege to user
REVOKE Takes back privileges granted from user
What is RDBMS?
RDBMS stands for Relational Database Management System . RDBMS is the basis for SQL, and for all modern database systems like MS SQL Server,
IBM DB2, Oracle, MySQL, and Microsoft Access.
A Relational database management system (RDBMS) is a database management system (DBMS) that is based on the relational model as introduced by E. F. Codd.

What is table?
The data in RDBMS is stored in database objects called tables. The table is a collection of related data entries and it consists of columns and rows.
Remember, a table is the most common and simplest form of data storage in a relational database. Following is the example of a CUSTOMERS table:


What is field?
Every table is broken up into smaller entities called fields. The fields in the CUSTOMERS table consist of ID, NAME, AGE, ADDRESS
and SALARY. A field is a column in a table that is designed to maintain specific information about every record in the table.

What is record or row?
A record, also called a row of data, is each individual entry that exists in a table. For example there are 7 records in the above CUSTOMERS table. Following is a single row of data or record in the CUSTOMERS table:



What is column?
A column is a vertical entity in a table that contains all information associated with a specific field in a table. For example, a column in the CUSTOMERS table is ADDRESS, which represents location description and would consist of the following:



What is NULL value?
A NULL value in a table is a value in a field that appears to be blank, which means a field with a NULL value is a field with no value.
It is very important to understand that a NULL value is different than a zero value or a field that contains spaces. A field with a NULL value
is one that has been left blank during record creation.
Constraints are the rules enforced on data columns on table. These are used to limit the type of data that can go into a table. This ensures the accuracy and reliability of the data in the database.
Constraints could be column level or table level. Column level constraints are applied only to one column where as table level constraints are applied to the whole table.
Following are commonly used constraints available in SQL :
• NOT NULL Constraint : Ensures that a column cannot have NULL value.
• DEFAULT Constraint : Provides a default value for a column when none is specified.
• UNIQUE Constraint : Ensures that all values in a column are different.
• PRIMARY Key : Uniquely identified each rows/records in a database table.
• FOREIGN Key : Uniquely identified a rows/records in any another database table.
• CHECK Constraint : The CHECK constraint ensures that all values in a column satisfy certain conditions.
• INDEX : Use to create and retrieve data from the database very quickly.
SQL SELECT Statement :
SELECT column1, column2....columnN
FROM   table_name;
SQL DISTINCT Clause :
SELECT DISTINCT column1, column2....columnN
FROM   table_name;
SQL WHERE Clause :
SELECT column1, column2....columnN
FROM   table_name
WHERE  CONDITION;
SQL AND/OR Clause :
SELECT column1, column2....columnN
FROM   table_name
WHERE  CONDITION-1 {AND|OR} CONDITION-2;
SQL IN Clause :
SELECT column1, column2....columnN
FROM   table_name
WHERE  column_name IN (val-1, val-2,...val-N);
SQL BETWEEN Clause :
SELECT column1, column2....columnN
FROM   table_name
WHERE  column_name BETWEEN val-1 AND val-2;
SQL LIKE Clause :
SELECT column1, column2....columnN
FROM   table_name
WHERE  column_name LIKE { PATTERN };
SQL ORDER BY Clause :
SELECT column1, column2....columnN
FROM   table_name
WHERE  CONDITION
ORDER BY column_name {ASC|DESC};
SQL GROUP BY Clause :
SELECT SUM(column_name)
FROM   table_name
WHERE  CONDITION
GROUP BY column_name;
SQL COUNT Clause :
SELECT COUNT(column_name)
FROM   table_name
WHERE  CONDITION;
SQL HAVING Clause :
SELECT SUM(column_name)
FROM   table_name
WHERE  CONDITION
GROUP BY column_name
HAVING (arithematic function condition);
SQL CREATE TABLE Statement :
CREATE TABLE table_name(
column1 datatype,
column2 datatype,
column3 datatype,
.....
columnN datatype,
PRIMARY KEY( one or more columns )
);
SQL DROP TABLE Statement :
DROP TABLE table_name;
SQL CREATE INDEX Statement :
CREATE UNIQUE INDEX index_name
ON table_name ( column1, column2,...columnN);
SQL DROP INDEX Statement :
ALTER TABLE table_name
DROP INDEX index_name;
SQL DESC Statement :
DESC table_name;
SQL TRUNCATE TABLE Statement :
TRUNCATE TABLE table_name;
SQL ALTER TABLE Statement :
ALTER TABLE table_name {ADD|DROP|MODIFY} column_name {data_ype};
SQL ALTER TABLE Statement (Rename) :
ALTER TABLE table_name RENAME TO new_table_name;
SQL INSERT INTO Statement :
INSERT INTO table_name( column1, column2....columnN)
VALUES ( value1, value2....valueN);
SQL UPDATE Statement :
UPDATE table_name
SET column1 = value1, column2 = value2....columnN=valueN
[ WHERE  CONDITION ];
SQL DELETE Statement :
DELETE FROM table_name
WHERE  {CONDITION};
SQL CREATE DATABASE Statement :
CREATE DATABASE database_name;
SQL DROP DATABASE Statement :
DROP DATABASE database_name;
SQL USE Statement :
USE database_name;
SQL COMMIT Statement :
COMMIT;
SQL ROLLBACK Statement :
ROLLBACK;
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