DDL Full Form

DDL stands for Data Definition Language. 
These commands are used to change the structure of a database and database objects.For example, DDL commands can be used to add, remove, or modify tables with in a database. 

The DDL commands are: 

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

1. CREATE :
This command is used to create table in the relational database . 
This can be done by specifying the names and datatypes of various columns. 

Syntax: 

CREATE TABLE TABLE_NAME
(
    column_name1 datatype1,
    column_name2 datatype2,
    column_name3 datatype3,
    column_name4 datatype4
);

The column_name in create table command will tell the name of the column and corresponding datatype will specify the datatype of that column.Here in this table the three column_names namely – Student_id is of type int ,Name is of type varchar and Marks is of type int. 
for example: 

CREATE TABLE 
Employee
(Student_id INT, 
Name VARCHAR(100), 
Marks INT); 

Student_id	Name	Marks

2. ALTER :
Alter command is used for altering the table in many forms like: 

1. Add a column
2. Rename existing column
3. Drop a column
4. Modify the size of the column or change datatype of the column

• ADD using ALTER –
  Syntax to add column : 

ALTER TABLE table_name ADD(
    column_name datatype); 

The above command will add a new column to the table.And the resulting table will have one more column like this: 

ALTER TABLE Student 
ADD
(Address  VARCHAR(200)); 

Here this command will add a new column “Address” in the table Student of datatype varchar(200); 

Student_id	Name	Marks	Address

• RENAME using ALTER –
  Syntax to rename column : 

ALTER TABLE 
table_name 
RENAME 
old_column_name TO new_column_name; 

The above command will rename the existing column to new column. 

ALTER TABLE 
Employee 
RENAME 
Marks TO Age; 

The command above will change the column_name from Marks to Age; 

Student_id	Name	Age	Address

• DROP using ALTER –
  Syntax to Drop a column : 

ALTER TABLE
table_name
DROp
(column_name); 

The above command will delete the existing column. 
For example: 

ALTER TABLE Employee 
DROP
(Age);  

Here the column_name =”Age”, has been deleted by this command; 

Student_id	Name	Address

• MODIFY using ALTER –
  Syntax to Modify a column : 

ALTER TABLE
Employee MODIFY
(column_name datatype); 

The above command will modify the existing column . 
For example: 

ALTER TABLE 
student 
MODIFY
(name varchar(300)); 

The above command will modify the column_name “Name” by changing the size of that column. 

Student_id	Name	Address

3. TRUNCATE :
This command removes all the records from a table. But this command will not destroy the table’s structure. 
Syntax :  

TRUNCATE TABLE table_name

This will delete all the records from the table.For example the below command will remove all the records from table student. 

Example: 

TRUNCATE TABLE Student; 

4. DROP :
This command completely removes the table from the database along with the destruction of the table structure. 

Syntax –

DROP TABLE table_name

This will delete all the records as well as the structure of the table. 
This is the main difference between TRUNCATE AND DROP.-TRUNCATE only removes the records whereas DROP completely destroys the table. 

Example:  

DROP TABLE Student; 

This command will remove the table records as well as destroys the schema too. 

This is all about the DDL commands. 

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