Adding

AUTO_INCREMENT

to any

varchar

column first requires the column to be converted into

int

CREATE TABLE note (
    title varchar(50),
    description varchar(50),
    PRIMARY KEY(title)
);
INSERT INTO `note` (`title`, `description`) VALUES ('first', 'value'), ('second', 'value');

Let's add

AUTO_INCREMENT

to

title

column. We are not able to add

AUTO_INCREMENT

without changing it's type to

int

ALTER TABLE note MODIFY title int NOT NULL AUTO_INCREMENT;

Note: In such cases If the table contains rows with string values, (for

AUTO INCREMENT

field) the string values will be replaced with numbers starting with 1