Autoincrement Id and key problem (MySQL)

Question

i have table and i tried to add

`id` int(11) NOT NULL auto_increment,

to table but i get

ERROR 1075: Incorrect table definition; There can only be one auto column and it must be defined as a key

So the questions are:

1. How I can find which one is the key column
2. Can I change it without problems and add the new ID field (with autoincrement)?

Answer 1

If the table is already created use ALTER:

ALTER TABLE `table` ADD COLUMN `id` INT(11) NOT NULL AUTO_INCREMENT, 
DROP PRIMARY KEY, 
ADD PRIMARY KEY (`id`)

If the table is being created you must set the auto_increment as primary key:

CREATE TABLE IF NOT EXISTS `database`.`table` (
`id`INT(11) NOT NULL AUTO_INCREMENT ,
`a` VARCHAR(45) NULL ,
`b` VARCHAR(255) NULL ,
`c` VARCHAR(45) NULL ,
PRIMARY KEY (`id`))
ENGINE = InnoDB
DEFAULT CHARACTER SET = utf8
COLLATE = utf8_unicode_ci

Answer 2

First backup the database. Then drop any foreign key associated with the table. truncate the foreign key table.Truncate the current table.Add the id with auto increment . Use sqlyog or workbench or heidisql or dbeaver or phpmyadmin. Then insert the table from backup using batch update.

Answer 3

`id` int(11) NOT NULL auto_increment primary key,

必须使其成为主键

Answer 4

ALTER TABLE `database`.`table_name` ADD COLUMN `new_column_name` INT NOT NULL AUTO_INCREMENT  AFTER `last_column_name_in_the_table` , CHANGE COLUMN `old_column_name` `old_column_name` INT(11) NOT NULL  

, DROP PRIMARY KEY 

, ADD PRIMARY KEY (`new_column_name`) ;

You can find the current primary key just by running show create table as matthewh suggested, and look for something like this in the output:

PRIMARY KEY ( old_column_name ),

Also you should check out MySQL Workbench if you can use it. It makes making changes like this really easy.

Answer 5

Doesnt anyone use unsigned integers for primary keys. And why bother using an optional display width ie int(11) when you're not even using zerofill !!

drop table if exists foo;
create table foo
(
 id int unsigned not null auto_increment primary key,
 ...
)
engine=innodb;