How to alter tabel auto increment mysql

CREATE TABLE ALLITEMS(
    itemid INT(10)UNSIGNED,
    itemname VARCHAR(50)
);

ALTER TABLE ALLITEMS CHANGE itemid itemid INT(10)AUTO_INCREMENT PRIMARY KEY;

DESC ALLITEMS;

INSERT INTO ALLITEMS(itemname)
VALUES
    ('Apple'),
    ('Orange'),
    ('Banana');

SELECT
    *
FROM
    ALLITEMS;

I was confused with CHANGE and MODIFY keywords before too:

ALTER TABLE ALLITEMS CHANGE itemid itemid INT(10)AUTO_INCREMENT PRIMARY KEY;

ALTER TABLE ALLITEMS MODIFY itemid INT(5);

While we are there, also note that AUTO_INCREMENT can also start with a predefined number:

ALTER TABLE tbl AUTO_INCREMENT = 100;

Source: http://stackoverflow.com/questions/6957370/alter-table-adding-autoincrement-in-mysql