unable to create constraint in mysql table

Question

I've got following tables:

node_sharing | CREATE TABLE `node_sharing` (
  `user_id` int(11) NOT NULL,
  `node_id` int(11) NOT NULL,
  PRIMARY KEY (`user_id`,`node_id`),
  KEY `fk_user_id_idx` (`user_id`),
  KEY `fk_node_id_idx` (`node_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8

and

users | CREATE TABLE `users` (
  `id` int(11) NOT NULL,
  `has_ard_access` tinyint(1) NOT NULL DEFAULT '0',
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8

I would like to create following foreign key with a constraint:

ALTER TABLE `node_sharing`
  ADD CONSTRAINT `fk_user_id`
  FOREIGN KEY (`user_id` )
  REFERENCES `users` (`id` )
  ON DELETE NO ACTION
  ON UPDATE NO ACTION;

and MySQL returns the following error:

ERROR 1005 (HY000): Can't create table 'MY_TABLE_NAME.#sql-4d0_218' (errno: 121)

What is wrong here?

PS node_sharing has been truncated, so there are no existing records that could disable putting the constraint on.

Answer 1

It is a duplicate key error. Did you have a table with same name before? If so, check InnoDB internal data dictionary. If not, check if you have another constraint with same name. Constraint names should be unique.