MySQL Error 1215: Cannot add foreign key constraint

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

 Answers

I'm guessing that Clients.Case_Number and/or Staff.Emp_ID are not exactly the same data type as Clients_has_Staff.Clients_Case_Numberand Clients_has_Staff.Staff_Emp_ID.

Perhaps the columns in the parent tables are INT UNSIGNED?

They need to be exactly the same data type in both tables.

---

For others same error may not always be due to column type mismatch, you can find out more information about a mysql foriegn key error by issuing command

SHOW ENGINE INNODB STATUS;

you may find a error near the top of the printed message something like

Cannot find an index in the referenced table where the referenced columns appear as the first columns, or column types in the table and the referenced table do not match for constraint.

---

In my case, I had deleted a table using SET FOREIGN_KEY_CHECKS=0, then SET FOREIGN_KEY_CHECKS=1 after. When I went to reload the table, I got error 1215. The problem was there was another table in the database that had a foreign key to the table I had deleted and was reloading. Part of the reloading process involved changing a data type for one of the fields, which made the foreign key from the other table invalid, thus triggering error 1215. I resolved the problem by dropping and then reloading the other table with the new data type for the involved field.

---

I got the same error while trying to add an fk. In my case the problem was caused by the FK table's PK which was marked as unsigned.

---

I had the same problem.
I solved it doing this:

I created the following line in the
primary key: (id int(11) unsigned NOT NULL AUTO_INCREMENT)

I found out this solution after trying to import a table in my schema builder. If it works for you, let me know!

Good luck!

Felipe Tércio

---

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 ;)

---

For MySQL (INNODB) ... get definitions for columns you want to link

SELECT * FROM information_schema.columns WHERE 
TABLE_NAME IN (tb_name','referenced_table_name') AND 
COLUMN_NAME  IN ('col_name','referenced_col_name')\G

compare and verify both column definitions have

same COLUMN_TYPE(length), same COLATION

could be helpfull to play like

set foreign_key_checks=0;
ALTER TABLE tb_name ADD FOREIGN KEY(col_name) REFERENCES ref_table(ref_column) ON DELETE ...
set foreign_key_checks=1;

---

Another reason: if you use ON DELETE SET NULL all columns that are used in the foreign key must allow null values. Someone else found this out in this question.

From my understanding it wouldn't be a problem regarding data integrity, but it seems that MySQL just doesn't support this feature (in 5.7).

---

For me it was the column types. BigINT != INT.

But then it still didn't work.

So I checked the engines. Make sure Table1 = InnoDB and Table = InnoDB

---

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.

---

I experienced this error for a completely different reason. I used MySQL Workbench 6.3 for creating my Data Model (awesome tool). I noticed that when the column order defined in the Foreign Key constraint definition does not fit the table column sequence this error is also generated.

It took me about 4 hours of trying everything else but checking that.

Now all is working well and I can get back to coding. :-)

This is a subtle version of what has already been said, but in my instance, I had 2 databases (foo and bar). I created foo first and I didn't realize it referenced a foreign key in bar.baz (which wasn't created yet). When I tried to create bar.baz (without any foreign keys), I kept getting this error. After looking around for a while I found the foreign key in foo.

So, long story short, If you get this error, you may have a pre-existing foreign key to the table being created.