Failed to add the foreign key - mysql

Question

I am trying to create two tables
CREATE TABLE Submissions
(
quiz_ID INT(2) UNSIGNED,
quiz_duration TIME,
student_ID INT(2) UNSIGNED,
attempt_date date,
PRIMARY KEY (quiz_ID, student_ID)
);
and
CREATE TABLE Students
(
student_ID INT(2) UNSIGNED,
student_name VARCHAR(100),
PRIMARY KEY (student_ID),
FOREIGN KEY (student_ID) REFERENCES Submissions(student_ID)
);
However, when I try to create the second table, it gives me an error
ERROR 1822 (HY000): Failed to add the foreign key constraint. Missing index for constraint 'students_ibfk_1' in the referenced table 'Submissions'
How do I fix it?

Answer 1

A foreign key must reference a primary key. You have defined the primary key of SUBMISSIONS as a compound key of (quiz_ID, student_ID). Consequently this does not match the foreign key reference of STUDENTS, where you attempt to reference a single column Submissions(student_ID). That's why you get the error.
But let's drill down into this. You tagged your question [database-design], and that was a good call because this issue arises from your data model.
A student can (presumably) participate in more than one quiz and therefore have more than one submission. This indicates that your data model is wrong. Whenever we have a 1:M relationship the 1 end (in this case STUDENTS) is referenced by a foreign key on the M end (i.e. SUBMISSIONS).
It also seems likely that your data model is missing a table for QUIZZES, and SUBMISSIONS should in fact be an intersection table.
Given all this, your tables should look like this:
CREATE TABLE Quizzes
(
quiz_ID INT(2) UNSIGNED,
quiz_duration TIME
PRIMARY KEY (quiz_ID)
);
CREATE TABLE Students
(
student_ID INT(2) UNSIGNED,
student_name VARCHAR(100),
PRIMARY KEY (student_ID)
);
CREATE TABLE Submissions
(
quiz_ID INT(2) UNSIGNED,
student_ID INT(2) UNSIGNED,
attempt_date date,
PRIMARY KEY (quiz_ID, student_ID),
FOREIGN KEY (quiz_ID) REFERENCES Quizzes(quiz_ID),
FOREIGN KEY (student_ID) REFERENCES Students(student_ID)
);

Answer 2

You have the foreign keys in the wrong order. The submissions should have the foreign key constraint. So:
CREATE TABLE Submissions (
quiz_ID INT(2) UNSIGNED,
quiz_duration TIME,
student_ID INT(2) UNSIGNED,
attempt_date date,
PRIMARY KEY (quiz_ID, student_ID),
CONSTRAINT fk_subsmissions_student_id FOREIGN KEY (student_id) REFERENCES students(student_id)
);
Of course, the Students table needs to be created, before the foreign key is defined.