Types of SQL Commands

There are five types of SQL commands: DDL, DML, DCL, TCL, and DQL.

Data Definition Language (DDL)

DDL changes the structure of the table like creating a table, deleting a table, altering a table, etc.

All the command of DDL are auto-committed that means it permanently save all the changes in the database.

Here are some commands that come under DDL:

1. CREATE
2. ALTER
3. DROP
4. TRUNCATE

CREATE

It is used to create a new table in the database.

Syntax:

CREATE TABLE TABLE_NAME (COLUMN_NAME DATATYPES[,....]);

Example:

CREATE TABLE EMPLOYEE(

Name VARCHAR2(20), 

Email VARCHAR2(100), 

DOB DATE

);

Drop

It is used to delete both the structure and record stored in the table.

Syntax:

DROP TABLE ;

Example:

DROP TABLE EMPLOYEE;

ALTER

It is used to alter the structure of the database. This change could be either to modify the characteristics of an existing attribute or probably to add a new attribute.

Syntax:

ALTER TABLE table_name ADD column_name COLUMN-definition; 

ALTER TABLE MODIFY (COLUMN DEFINITION....);

Example:

ALTER TABLE STU_DETAILS ADD(ADDRESS VARCHAR2(20));

ALTER TABLE STU_DETAILS MODIFY (NAME VARCHAR2(20)); 

TRUNCATE

It is used to delete all the rows from the table and free the space containing the table.

Syntax:

TRUNCATE TABLE table_name;

Example: 

TRUNCATE TABLE EMPLOYEE;

Data Manipulation Language

DML commands are used to modify the database. It is responsible for all form of CHANGES in the database.

The command of DML is not auto-committed that

means it can't permanently save all the changes in the database. They can be rollback.

Here are some commands that come under DML:

• INSERT
• UPDATE
• DELETE

INSERT

The INSERT statement is a SQL query. It is used to insert data into the row of a table.

Syntax:

INSERT INTO TABLE_NAME (col1, col2, col3,.... col N)

VALUES (value1, value2, value3, .... valueN);

OR

INSERT INTO TABLE_NAME                      

VALUES (value1, value2, value3, .... valueN);

Example:

INSERT INTO XYZ (Author, Subject) VALUES ("Sonoo", "DBMS");   

UPDATE

Update: This command is used to update or modify the value of a column in the table.

Syntax:

UPDATE table_name 

SET [column_name1= value1,...column_n ameN = valueN] 

WHERE CONDITION;

Example:

UPDATE students

SET User_Name = 'Sonoo'

WHERE Student_Id = '3' 

Data Control Language

DCL commands are used to GRANT and TAKE BACK authority from any database user.

Here are some commands that come under DCL:

• Grant
• Revoke

Grant

It is used to give user access privileges to a database.

Example:

GRANT SELECT, UPDATE ON MY_TABLE TO SOME_USER, ANOT HER_USER;

Revoke

It is used to take back permissions from the user.

Example:

REVOKE SELECT, UPDATE ON MY_TABLE FROM USER1, USER2;

Transactions Control Language

TCL commands can only use with DML commands like INSERT, DELETE and UPDATE only.

These operations are automatically committed in the database that's why they cannot be used while creating tables or dropping them.

Here are some commands that come under TCL:

1. COMMIT
2. ROLLBACK
3. SAVEPOINT

COMMIT

Commit command is used to save all the transactions to the database.

Syntex:

COMMIT; 

Example:

DELETE FROM CUSTOMERS
WHERE AGE = 25;
COMMIT;

Rollback

Rollback command is used to undo transactions that have not already been saved to the database.

Syntex:

Rollback;

SAVEPOINT

it is used to roll the transaction back to a certain point without rolling back the entire transaction.

Syntex:

ROLLBACK;
SAVEPOINT SAVEPOINT_NAME;

Example:

DELETE FROM CUSTOMERS
WHERE AGE = 25;
ROLLBACK;