INTRODUCTION
Structured Query Language (SQL) as we all know is the database language by the use of which we can perform certain operations on the existing database and also we can use this language to create a database. SQL uses certain commands like Create, Drop, Insert, etc. to carry out the required tasks.
These
SQL commands are mainly categorized into four categories as:
- DDL – Data Definition Language
- DQL – Data Query Language
- DML – Data Manipulation Language
- DCL – Data Control Language
- TCL – Transaction Control Language
1) DDL (Data Definition Language):
DDL
or Data Definition Language actually consists of the SQL commands that can be
used to define the database schema. It simply deals with descriptions of the
database schema and is used to create and modify the structure of database
objects in the database. DDL is a set of SQL commands used to create, modify,
and delete database structures but not data. These commands are normally not
used by a general user, who should be accessing the database via an
application.
# List of DDL commands:
CREATE:
This command is used to create the database or its objects (like table, index,
function, views, store procedure, and triggers).
Example:
CREATE DATABASE <db_name>
CREATE TABLE <tbl_name>
(
             column1
datatype1,
             column2
datatype2,
……
columnN datatypeN
)
DROP:
This command is used to delete objects from the database.
Example:
DROP DATABASE <db_name>;
DROP TABLE <tbl_name>;
ALTER:
This is used to alter the structure of the database.
Example:
ALTER TABLE <tbl_name> ADD
column_name datatype;
TRUNCATE:
This is used to remove all records from a table, including all spaces allocated
for the records are removed.
Example:
TRUNCATE TABLE <tbl_name>
COMMENT:
This is used to add comments to the data dictionary.
Example:
-- SELECT *FROM <tbl_name>Â // single line comment
/*
// multi line comment
*/
RENAME:
This is used to rename an object existing in the database.
Example:
2) DQL (Data Query Language):
DQL
statements are used for performing queries on the data within schema objects.
The purpose of the DQL Command is to get some schema relation based on the
query passed to it. We can define DQL as follows it is a component of SQL
statement that allows getting data from the database and imposing order upon
it. It includes the SELECT statement. This command allows getting the data out
of the database to perform operations with it. When a SELECT is fired against a
table or tables the result is compiled into a further temporary table, which is
displayed or perhaps received by the program i.e. a front-end.
# List of DQL:
SELECT:
It is used to retrieve data from the database.
Example:
SELECT *FROM <tbl_name> // this
query gets all data from the table
SELECT column_name1, column_name2 FROM
<tbl_name> // gets specific column data
3) DML (Data Manipulation Language):
The
SQL commands that deals with the manipulation of data present in the database
belong to DML or Data Manipulation Language and this includes most of the SQL
statements. It is the component of the SQL statement that controls access to
data and to the database. Basically, DCL statements are grouped with DML
statements.
# List of DML commands:
INSERT:
It is used to insert data into a table.
Example:
INSERT INTO <tbl_name> (column1,column2,column3…,columnN)
VALUES(‘value1’,’value2’,’value3’,…,’valueN’)
// multiple insert
INSERT INTO <tbl_name> (column1,column2,column3…,columnN)
VALUES(‘value1’,’value2’,’value3’,…,’valueN’), ((‘value1’,’value2’,’value3’,…,’valueN’)
UPDATE:
It is used to update existing data within a table.
Example:
// updates the whole table data
UPDATE <tbl_name> SET column1 = ‘value1’,
column2=’value2’,…., column=’valueN’
// updates the table data of specific
id or value
UPDATE <tbl_name> SET column1 = ‘value1’,
column2=’value2’,…., column=’valueN’  WHERE column = ‘somvalue’
DELETE:
It is used to delete records from a database table.
Example:
DELETE FROM <tbl_name> WHERE some_column
= ‘somevalue’
4) DCL (Data Control Language):
DCL
includes commands such as GRANT and REVOKE which mainly deal with the rights,
permissions, and other controls of the database system.
# List of DCL commands:
GRANT:
This command gives users access privileges to the database.
Example:
GRANT privilege_name ON object_name TO
{user_name |PUBLIC |role_name}
[WITH GRANT OPTION];
REVOKE:
This command withdraws the user’s access privileges given by using the GRANT
command.
Example:
REVOKE privilege_name ON object_name FROM
{user_name |PUBLIC |role_name}
5) TCL (Transaction Control Language):
Transactions
group a set of tasks into a single execution unit. Each transaction begins with
a specific task and ends when all the tasks in the group successfully complete.
If any of the tasks fail, the transaction fails. Therefore, a transaction has
only two results: success or failure. You can explore more about transactions
here. Hence, the following TCL commands are used to control the execution of a
transaction:
# List of TCL Commands
COMMIT:
Commits a Transaction.
ROLLBACK:
Rollbacks a transaction in case of any error occurs.
SAVEPOINT:
Sets a save point within a transaction.
SET
TRANSACTION: Specifies characteristics for the transaction.