Auto Increment

Auto increment is used to  generate an unique, when a new record is inserted into a table.
If use a auto increment than increase by 1. 
In table sequence is increment automatically.
Auto increment ignore null value.

Syntax :-

CREATE TABLE table_name
(
Column_name int NOT NULL AUTO_INCREMENT,
Column_name1 varchar (50) NOT NULL,
Column_name2 varchar (50),
PRIMARY KEY (column_name)
);

Example :-

CREATE TABLE emp
(
Emp_id int NOT NULL AUTO_INCREMENT,
Emp_name varchar (50) NOT NULL,
City varchar (50),
PRIMARY KEY (Emp_id)
);

Insert rule are different 

INSERT INTO emp (emp_name, city)
VALUES
('Subham', 'Delhi'),
('Ankit', 'Mumbai');

INSERT INTO emp (emp_id, emp_name, city)
VALUES
(NULL, 'Subham', 'Delhi'),
(NULL, 'Ankit', 'Mumbai');

INSERT INTO emp (emp_id, emp_name, city)
VALUES
(NULL, 'Subham', 'Delhi'),
(1, 'Ankit', 'Mumbai');

Table:-

Use NULL VALUE