Data Definition Commands in DBMS

Data definition commands are used to create, modify and remove database objects such as schemas, tables, views, indexes etc.

Common Data Definition commands −

Create

The main use of create command is to create a new table in database. It has a predefined syntax in which we specify the columns and their respective data types.

syntax

CREATE TABLE <TABLE NAME>
( <COLUMN NAME>  <DATA TYPE>,
<COLUMN NAME>  <DATA TYPE>,
<COLUMN NAME>  <DATA TYPE>,
<COLUMN NAME>  <DATA TYPE>
);

Example  

Create a student table with columns student name and roll number.

CREATE TABLE STUDENT
(STUDENT_NAME VARCHAR(30),
ROLL_NUMBER INT
);

Alter

An existing database object can be modified using the alter command. Alter command can do following changes to any table-

• Add new columns.

• Add new integrity constraints.

• Modify existing columns. 

• Drop integrity constraints.

Syntax

General Syntax of the ALTER command is mentioned below −

For adding a new column

ALTER TABLE <table_name>  ADD  <column_name>

 For renaming a table

ALTER TABLE <table_name>  RENAME To <new_table_name >

For modifying a column

ALTER TABLE <table_name> MODIFY <column_name > <data type >

For deleting a column

ALTER TABLE <table_name> DROP COLUMN <column_name>

Drop

This command can delete an index, table or view. Basically, any component from a relational database management system can be removed using the  Drop command. Once the object is dropped, it cannot be reused.

The general syntax of drop command is as follows −

DROP TABLE <table_name>;
DROP DATABASE <database_name>;
DROP TABLE <index_name>;

Truncate

Using the truncate command, all the records in a database are deleted, but the database structure is maintained.

syntax

TRUNCATE TABLE <table name>

Comment

This command is used to add comments to the data dictionary.

syntax

• Single line comments:  use ‘ --‘ before any text. 
• Multiline comments:  /* comments in between   */

Rename

The rename command renames an object

Syntax

Rename <old name> to <new name>