Cannot add foreign key constraint 4

Question

Error 1215: Cannot add foreign key constraint

CREATE TABLE IF NOT EXISTS `mydb`.`proba` (
  `key` VARCHAR(255) NOT NULL,
  `value` VARCHAR(255) NOT NULL,
  PRIMARY KEY (`key`, `value`))
ENGINE = InnoDB;


CREATE TABLE IF NOT EXISTS `mydb`.`zavisnost` (
  `key` VARCHAR(255) NOT NULL,
  `value` VARCHAR(255) NOT NULL,
  PRIMARY KEY (`key`, `value`),
  CONSTRAINT `fk_zavisnost_proba1`
    FOREIGN KEY (`key`)
    REFERENCES `mydb`.`proba` (`value`)
    ON DELETE NO ACTION
    ON UPDATE NO ACTION)
ENGINE = InnoDB;

I don't understand why this error appears. Please someone help... Thanks in advance..

Answer 1

To define a foreign key on a column its parent referenced field must be indexed.

As per documentation on foreign key constraints:

REFERENCES tbl_name (index_col_name,...)

Define an INDEX on value in table proba and it should be working.

CREATE TABLE IF NOT EXISTS `mydb`.`proba` (
  `key` VARCHAR(255) NOT NULL,
  `value` VARCHAR(255) NOT NULL,

  KEY ( `value` ), -- <-------- this line was missing

  PRIMARY KEY (`key`, `value`))
ENGINE = InnoDB;

Once you define a KEY on the parent column, a foreign key column in zavisnost table can refer it, like.

...
FOREIGN KEY (`key`)
REFERENCES `mydb`.`proba` (`value`)
...

Refer to:

• MySQL Using FOREIGN KEY Constraints

[CONSTRAINT [symbol]] FOREIGN KEY
[index_name] (index_col_name, ...)
REFERENCES tbl_name (index_col_name,...)
[ON DELETE reference_option]
[ON UPDATE reference_option]

reference_option:
RESTRICT | CASCADE | SET NULL | NO ACTION