DDL stands for Data Defination 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); 

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

1. 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

2. 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

3. 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

4. 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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