MySQL Error 1215: Cannot add foreign key constrain

Question

I am trying to forward engineer my new schema onto my db server, but I can't figure out why I am getting this error. I've tried to search for the answer here, but everything I've found has said to either set the db engine to Innodb or to make sure the keys I'm trying to use as a foreign key are primary keys in their own tables. I have done both of these things, if I'm not mistaken. Any other help you guys could offer?

Executing SQL script in server

ERROR: Error 1215: Cannot add foreign key constraint

-- -----------------------------------------------------
-- Table `Alternative_Pathways`.`Clients_has_Staff`
-- -----------------------------------------------------

CREATE  TABLE IF NOT EXISTS `Alternative_Pathways`.`Clients_has_Staff` (
  `Clients_Case_Number` INT NOT NULL ,
  `Staff_Emp_ID` INT NOT NULL ,
  PRIMARY KEY (`Clients_Case_Number`, `Staff_Emp_ID`) ,
  INDEX `fk_Clients_has_Staff_Staff1_idx` (`Staff_Emp_ID` ASC) ,
  INDEX `fk_Clients_has_Staff_Clients_idx` (`Clients_Case_Number` ASC) ,
  CONSTRAINT `fk_Clients_has_Staff_Clients`
    FOREIGN KEY (`Clients_Case_Number` )
    REFERENCES `Alternative_Pathways`.`Clients` (`Case_Number` )
    ON DELETE NO ACTION
    ON UPDATE NO ACTION,
  CONSTRAINT `fk_Clients_has_Staff_Staff1`
    FOREIGN KEY (`Staff_Emp_ID` )
    REFERENCES `Alternative_Pathways`.`Staff` (`Emp_ID` )
    ON DELETE NO ACTION
    ON UPDATE NO ACTION)
ENGINE = InnoDB

SQL script execution finished: statements: 7 succeeded, 1 failed

Here is the SQL for the parent tables.

CREATE  TABLE IF NOT EXISTS `Alternative_Pathways`.`Clients` (
  `Case_Number` INT NOT NULL ,
  `First_Name` CHAR(10) NULL ,
  `Middle_Name` CHAR(10) NULL ,
  `Last_Name` CHAR(10) NULL ,
  `Address` CHAR(50) NULL ,
  `Phone_Number` INT(10) NULL ,
  PRIMARY KEY (`Case_Number`) )
ENGINE = InnoDB

CREATE  TABLE IF NOT EXISTS `Alternative_Pathways`.`Staff` (
  `Emp_ID` INT NOT NULL ,
  `First_Name` CHAR(10) NULL ,
  `Middle_Name` CHAR(10) NULL ,
  `Last_Name` CHAR(10) NULL ,
  PRIMARY KEY (`Emp_ID`) )
ENGINE = InnoDB

Answer 1

When this error occurrs because the referenced table uses the MyISAM engine this answer provides a quick way to convert your database so all Django model tables use InnoDB: https://stackoverflow.com/a/15389961/2950621

It's a Django management command called convert_to_innodb.

Answer 2

This also happens when the type of the columns is not the same.

e.g. if the column you are referring to is UNSIGNED INT and the column being referred is INT then you get this error.

Answer 3

i had the same issue, my solution:

Before:

CREATE TABLE EMPRES
( NoFilm smallint NOT NULL

  PRIMARY KEY (NoFilm)

  FOREIGN KEY (NoFilm) REFERENCES cassettes

);

Solution:

CREATE TABLE EMPRES
(NoFilm smallint NOT NULL REFERENCES cassettes,

 PRIMARY KEY (NoFilm)

);

I hope it's help ;)

Answer 4

Wooo I just got it ! It was a mix of a lot of already posted answers (innoDB, unsigned, etc). One thing I didn't see here though is : if your FK is pointing on a PK, ensure the source column has a value that makes sense. For example, if the PK is a mediumint(8), make sure the source column also contains a mediumint(8). That was part of the problem for me.

Answer 5

Reasons you may get a foreign key constraint error:

1. You are not using InnoDB as the engine on all tables.
2. You are trying to reference a nonexistent key on the target table. Make sure it is a key on the other table (it can be a primary or unique key)
3. The types of the columns are not the same (exception is the column on the referencing table can be nullable).
4. If the PK/FK is a varchar make sure the collation is the same for both.

Update:

5. One of the reasons may also be that the column you are using for ON DELETE SET NULL is not defined to be null. So make sure that the column is set default null.

Check these.

Answer 6

Even i had the same problem . And the fault was with the "unsigned" marker in the FK's table PK