why does this foreign key fail

Question

CREATE TABLE `fitness_pledges` (
    `ID` INT(10) UNSIGNED NOT NULL AUTO_INCREMENT,
    `first_name` VARCHAR(250) NOT NULL,
    `last_name` VARCHAR(250) NOT NULL,
    `email` VARCHAR(100) NOT NULL,
    `token` BIGINT(20) UNSIGNED NOT NULL,
    `email_valid` TINYINT(1) UNSIGNED NOT NULL DEFAULT '0',
    `entered_sweeps` TINYINT(1) UNSIGNED NOT NULL DEFAULT '0',
    `referrals` INT(10) UNSIGNED NOT NULL DEFAULT '0',
    PRIMARY KEY (`ID`),
    UNIQUE INDEX `email` (`email`),
    UNIQUE INDEX `token` (`token`)
)
COMMENT='holds validation token, first name, last name, email address, number of referrals and whether sweepstakes was entered\r\n'
COLLATE='latin1_swedish_ci'
ENGINE=InnoDB
;

CREATE TABLE `finess_referrals` (
    `ID` INT(10) UNSIGNED NOT NULL AUTO_INCREMENT,
    `first_name` VARCHAR(250) NOT NULL,
    `last_name` VARCHAR(250) NOT NULL,
    `email` VARCHAR(100) NOT NULL,
    `token` BIGINT(20) UNSIGNED NOT NULL,
    `referral_token` BIGINT(20) UNSIGNED NOT NULL,
    `referral_validated` TINYINT(4) NOT NULL DEFAULT '0',
    PRIMARY KEY (`ID`),
    INDEX `token_FK` (`token`),
    INDEX `referral_token_FK` (`referral_token`),
    CONSTRAINT `referral_token_FK` FOREIGN KEY (`referral_token`) REFERENCES `fitness_pledges` (`token`) ON DELETE NO ACTION,
    CONSTRAINT `token_FK` FOREIGN KEY (`token`) REFERENCES `fitness_pledges` (`token`) ON DELETE NO ACTION
)
COMMENT='holds the first name, last name, email and validation token of the referrer as well as the token of the referred individual and a flag for whether the referred individual\'s email has been validated'
COLLATE='latin1_swedish_ci'
ENGINE=InnoDB
;

This Alter Table, generated by HeidiSQL, attempting to add a Foreign Key for the combined first_name, last_name and email fields fails with Foreign key constraint is incorrectly formed. The fields are all defined the same, the collation is the same. Why is it failing?

ALTER TABLE `finess_referrals`
ADD CONSTRAINT `first_last_email_FK` FOREIGN KEY (`first_name`, `last_name`, `email`) 
REFERENCES `fitness_pledges` (`first_name`, `last_name`, `email`) ON DELETE NO ACTION;

Answer 1

You need an index in the fitness_pledges table for (first_name, last_name, email) first. From Using FOREIGN KEY Constraints:

MySQL requires indexes on foreign keys and referenced keys so that foreign key checks can be fast and not require a table scan.

It will create an index in the referencing table automatically if necessary, but it doesn't do so for the referenced table.

So you need to do:

CREATE INDEX first_last_email ON fitness_pledges (first_name, last_name, email);