ALTER table - adding AUTOINCREMENT in 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;

ALTER TABLE `ALLITEMS`
    CHANGE COLUMN `itemid` `itemid` INT(10) UNSIGNED NOT NULL AUTO_INCREMENT;

The syntax:

   ALTER TABLE `table1` CHANGE `itemId` `itemId` INT( 11 ) NOT NULL AUTO_INCREMENT 

But the table needs a defined key (ex primary key on itemId).