ALTER table - adding AUTOINCREMENT in MySQL

Question

I created a table in MySQL with on column itemID.

After creating the table, now I want to change this column to AUTOINCREMENT.

How can this be done using ALTER statements?

Table definition:

ALLITEMS (itemid int(10) unsigned, itemname varchar(50))

I am using the following code but it is throwing a syntax error

Error: syntax incorrect.

ALTER TABLE allitems
MODIFY itemid INT(10) UNSIGNED AUTOINCREMENT; 

Answer 1

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;