Can not Add External Index Constraints - mysql

Question

This is the exact error that WAMP returns when I run the child code from an external file called entries.txt
ERROR 1215 (HY000): Can not Add External Index Constraints
I need to be able to connect the parent tables to the child table so that links can be made between the tables easily.
The question states:
Create and populate a third table called entries, again using query scripts.
This table should contain foreign keys to allow sensible links to be
made with the other two tables, together with the dates of each exam.
Parent Tables:
CREATE TABLE IF NOT EXISTS students(
student_id INT UNSIGNED NOT NULL AUTO_INCREMENT,
first_name VARCHAR(20) NOT NULL,
middle_name VARCHAR(20),
last_name VARCHAR(40) NOT NULL,
email VARCHAR(60) NOT NULL,
password CHAR(40) NOT NULL,
reg_date DATETIME NOT NULL,
PRIMARY KEY (student_id),
UNIQUE (email));
CREATE TABLE IF NOT EXISTS subjects(
subject_id INT UNSIGNED NOT NULL AUTO_INCREMENT,
subject_name CHAR(30) NOT NULL,
level_of_entry VARCHAR(10) NOT NULL,
exam_board CHAR(20) NOT NULL,
PRIMARY KEY (subject_id));
Child Table:
CREATE TABLE IF NOT EXISTS entries(
date_of_exam DATETIME NOT NULL,
first_name VARCHAR,
middle_name VARCHAR,
last_name VARCHAR,
subject_name CHAR,
level_of_entry VARCHAR,
exam_board CHAR,
INDEX idx_first_name(first_name),
INDEX idx_middle_name(middle_name),
INDEX idx_last_name(last_name),
INDEX idx_subject_name(subject_name),
INDEX idx_level_of_entry(level_of_entry),
INDEX idx_exam_board(exam_board),
PRIMARY KEY (date_of_exam),
CONSTRAINT fk_first_name FOREIGN KEY (first_name) REFERENCES students(first_name),
CONSTRAINT fk_middle_name FOREIGN KEY (middle_name) REFERENCES students(middle_name),
CONSTRAINT fk_last_name FOREIGN KEY (last_name) REFERENCES students(last_name),
CONSTRAINT fk_subject_name FOREIGN KEY (subject_name) REFERENCES subjects(subject_name),
CONSTRAINT fk_level_of_entry FOREIGN KEY (level_of_entry) REFERENCES subjects(level_of_entry),
CONSTRAINT fk_exam_board FOREIGN KEY (exam_board) REFERENCES subjects(exam_board)
)ENGINE=InnoDB;

Answer 1

A foreign key in the child table must reference the primary key of the parent table only. You don't need to create clones of every attribute column of the parent tables, just the primary key column. If you need to read the other attributes, you'd write a query with a JOIN.
CREATE TABLE IF NOT EXISTS entries(
student_id INT UNSIGNED NOT NULL,
subject_id INT UNSIGNED NOT NULL,
date_of_exam DATETIME NOT NULL,
PRIMARY KEY (student_id, subject_id, date_of_exam),
CONSTRAINT fk_student FOREIGN KEY (student_id) REFERENCES students(student_id),
CONSTRAINT fk_subject FOREIGN KEY (subject_id) REFERENCES subjects(subject_id),
)ENGINE=InnoDB;