MySql can't make column auto_increment

This will happen if the table contains an existing record with an id of 0 (or negative). Updating all existing records to use positive values will allow auto_increment to be set on that column.

Edit: Some people asked how that 0 got in there. For clarification, the MySQL Reference Manual states that "For numeric types, the default is 0, with the exception that for integer or floating-point types declared with the AUTO_INCREMENT attribute, the default is the next value in the sequence." So, if you performed an insert on a table without providing a value for the numeric column before the auto_increment was enabled, then the default 0 would be used during the insert. More details may be found at https://dev.mysql.com/doc/refman/5.0/en/data-type-defaults.html.

I also had this issue when trying to convert a column to auto_increment where one row had a value of 0. An alternative to changing the 0 value temporarily is via setting:

SET SESSION sql_mode='NO_AUTO_VALUE_ON_ZERO';

for the session.

This allowed the column to be altered to auto_increment with the zero id in place.

The zero isn't ideal - and I also wouldn't recommend it being used in an auto_increment column. Unfortunately it's part of an inherited data set so I'm stuck with it for now.

Best to clear the setting (and any others) afterwards with:

SET SESSION sql_mode='';

although it will be cleared when the current client session clsoes.

Full details on the 'NO_AUTO_VALUE_ON_ZERO' setting here.

This happens when MySQL can not determine a proper auto_increment value. In your case, MySQL choose 1 as next auto_increment value, however there is already row with that value in the table.

One way to resolve the issue is to choose a proper auto_increment value yourself:

ALTER TABLE ... CHANGE COLUMN `Id` `Id` INT(11) NOT NULL AUTO_INCREMENT, AUTO_INCREMENT = 123456;

(Note the AUTO_INCREMENT=123456 at the end.)