Foreign key constraint creation fails with MySQL Error 1215

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1

I’m encountering an issue with MySQL where I can’t establish a foreign key relationship. The error I’m getting states, “Cannot add foreign key constraint,” along with the error code 1215.

I’ve looked up common solutions and ensured that both of the involved tables are using the InnoDB engine. Additionally, I’ve confirmed that the columns being referenced are indeed primary keys in their corresponding parent tables. Despite these checks, I’m still uncertain about what is causing the problem.

Here’s the junction table I’m attempting to set up:

CREATE TABLE IF NOT EXISTS `Company_DB`.`Users_has_Roles` (
  `Users_ID` INT NOT NULL,
  `Roles_Role_ID` INT NOT NULL,
  PRIMARY KEY (`Users_ID`, `Roles_Role_ID`),
  INDEX `idx_users_roles_role` (`Roles_Role_ID` ASC),
  INDEX `idx_users_roles_user` (`Users_ID` ASC),
  CONSTRAINT `fk_users_roles_users`
    FOREIGN KEY (`Users_ID`)
    REFERENCES `Company_DB`.`Users` (`ID`)
    ON DELETE CASCADE
    ON UPDATE CASCADE,
  CONSTRAINT `fk_users_roles_roles`
    FOREIGN KEY (`Roles_Role_ID`)
    REFERENCES `Company_DB`.`Roles` (`Role_ID`)
    ON DELETE CASCADE
    ON UPDATE CASCADE)
ENGINE = InnoDB;

And these are the parent tables:

CREATE TABLE IF NOT EXISTS `Company_DB`.`Users` (
  `ID` INT NOT NULL,
  `Username` VARCHAR(20) NULL,
  `Email` VARCHAR(50) NULL,
  `Full_Name` VARCHAR(100) NULL,
  `Department` VARCHAR(30) NULL,
  PRIMARY KEY (`ID`))
ENGINE = InnoDB;

CREATE TABLE IF NOT EXISTS `Company_DB`.`Roles` (
  `Role_ID` INT NOT NULL,
  `Role_Name` VARCHAR(25) NULL,
  `Description` VARCHAR(100) NULL,
  `Permission_Level` INT NULL,
  PRIMARY KEY (`Role_ID`))
ENGINE = InnoDB;

What else should I check to troubleshoot this foreign key constraint error?

2

Check your character set and collation settings on those referenced columns. I hit this exact issue - parent table columns used utf8mb4_unicode_ci but the foreign key columns defaulted to utf8mb4_general_ci. Even with INT types, MySQL gets picky about collation matching depending on your config. Run SHOW CREATE TABLE on both parent tables and make sure collations match exactly. Also got burned by different SQL modes when creating tables vs adding constraints. Try SET foreign_key_checks = 0, create your constraint, then SET foreign_key_checks = 1. Really helped me pinpoint which constraint was actually failing.

3

Had this exact problem last month - spent hours pulling my hair out! Your primary key columns probably don’t have AUTO_INCREMENT defined. MySQL is picky about data type matching, including attributes like AUTO_INCREMENT and UNSIGNED. Foreign keys won’t work without perfect matches. Add AUTO_INCREMENT to your primary keys in both parent tables. Also, create parent tables before the junction table - I screwed this up once and got the same 1215 error. MySQL needs the referenced tables to exist first. One more thing: check if you’ve got existing data that breaks the constraint. Even with IF NOT EXISTS, bad data will kill the constraint creation.

4

Check for existing data in the junction table that breaks the constraint - even ‘empty’ tables can have orphaned records hanging around. If that doesn’t work, try dropping and recreating the whole schema in the right order (parent tables first, then junction). I’ve seen MySQL cache old table definitions that screw up FK creation. Also make sure both ID columns match exactly - not just INT, but the same signed/unsigned attributes.