php database table creation fails - php

Question

I use the following gist to make an OOP attempt to create a database connection:
https://gist.github.com/jonashansen229/4534794
It seems to work so far.
But the creation of the database table passed_exams fails.
Edit:
After recent comments and suggestions i updated my code:
require_once 'Database.php'; // the gist 4534794
class DatabaseSchema {
public function createStudents() {
$db = Database::getInstance();
$mysqli = $db->getConnection();
$create_students = 'CREATE TABLE IF NOT EXISTS students (
id INT(6) UNSIGNED NOT NULL AUTO_INCREMENT PRIMARY KEY,
firstname VARCHAR(40) NOT NULL,
lastname VARCHAR(40) NOT NULL,
university VARCHAR(50)
)';
$result = $mysqli->query($create_students);
}
public function createPassedExams() {
$db = Database::getInstance();
$mysqli = $db->getConnection();
$create_passed_exams = 'CREATE TABLE IF NOT EXISTS passed_exams (
id INT(6) UNSIGNED NOT NULL AUTO_INCREMENT PRIMARY KEY,
name VARCHAR(40) NOT NULL,
student_id INT(6),
FOREIGN KEY (student_id) REFERENCES students(id) ON DELETE CASCADE
)';
$result = $mysqli->query($create_passed_exams);
}
}
$db_student = new DatabaseSchema();
$db_student->createStudents();
$db_student->createPassedExams();
When i look in the mysql console, only table students is created.
Why is table passed_exams missing?

Answer 1

You create the string to check $query_students = 'SELECT ID FROM STUDENTS';
but you never actually run this. Then u check the string if it is Empty, it will never be empty in your code.
What you should do is use the CREATE ... IF NOT EXISTS syntax of mysql, and not what you do here.
First example show the syntax https://dev.mysql.com/doc/refman/5.5/en/create-table.html

Answer 2

The id column on your students table is INT(6) UNSIGNED but the student_id column on the passed_exams table is a signed INT(6). Therefore the FOREIGN KEY (student_id) REFERENCES students(id) ON DELETE CASCADE clause will fail with "Error Code: 1215. Cannot add foreign key constraint".
I advise you to implement some error handling so that you would see this error message rather than blindly continue executing code.