MySql can't make column auto_increment

Question

I have a table "Bestelling" with 4 columns: "Id" (PK), "KlantId", "Datum", "BestellingsTypeId", now I want to make the column Id auto_increment, however, when I try to do that, I get this error:

ERROR 1062: ALTER TABLE causes auto_increment resequencing, resulting in duplicate entry '1' for key 'PRIMARY'  SQL Statement:  ALTER TABLE `aafest`.`aafest_bestelling` CHANGE COLUMN `Id` `Id` INT(11) NOT NULL AUTO_INCREMENT    ERROR: Error when running failback script. Details follow.    ERROR 1046: No database selected  SQL Statement:  CREATE TABLE `aafest_bestelling` (    `Id` int(11) NOT NULL,    `KlantId` int(11) DEFAULT NULL,    `Datum` date DEFAULT NULL,    `BestellingstypeId` int(11) DEFAULT NULL,    PRIMARY KEY (`Id`)  ) ENGINE=InnoDB DEFAULT CHARSET=latin1 

Anyone got an idea?

Answer 1

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.

Answer 2

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.