Why can't I add foreign key constraints - mysql

Question

this is my code:
CREATE DATABASE exams;
SHOW DATABASES;
CREATE TABLE IF NOT EXISTS students(
student_id INT UNSIGNED NOT NULL AUTO_INCREMENT,
first_name VARCHAR(20) NOT NULL,
middle_name VARCHAR(40),
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));
SHOW table status
INSERT INTO exams_3121(student_id, first_name, middle_name, last_name, email, password, reg_date)
CREATE TABLE entries
(
entrie_id int NOT NULL,
student_id int NOT NULL,
subject_id int,
PRIMARY KEY (entrie_id),
FOREIGN KEY (student_id) REFERENCES student(student_id),
FOREIGN KEY (subject_id)REFERENCES subject(subject_id)
)
CREATE DATABASE subjects;
CREATE TABLE IF NOT EXISTS subjects(
subject_id INT UNSIGNED NOT NULL AUTO_INCREMENT,
subject_name VARCHAR(20) NOT NULL,
level_entery VARCHAR(40)NOT NULL,
exam_board VARCHAR(60) NOT NULL,
PRIMARY KEY (subject_id));
CREATE TABLE entries
(
entrie_id int NOT NULL,
entrie_id int NOT NULL,
subject_id int,
PRIMARY KEY (entrie_id),
FOREIGN KEY (student_id) REFERENCES student(student_id),
FOREIGN KEY (subject_id)REFERENCES subject(subject_id)
)
When I use this code it says cannot add foreign key constraint
and I don't know what to do. Please and thanks in advance.

Answer 1

There are two problems.
First, you got the names of the table you're referencing wrong. The name of the tables are students and subjects, but you wrote student and subject.
Second, the entries table has two entrie_id columns. One of them should be student_id.
CREATE TABLE entries
(
entrie_id int NOT NULL,
student_id int NOT NULL,
subject_id int,
PRIMARY KEY (entrie_id),
FOREIGN KEY (student_id) REFERENCES students(student_id),
FOREIGN KEY (subject_id) REFERENCES subjects(subject_id)
)
Also, if you're creating multiple databases and putting your tables in them, you'll need to refer to tables with their database prefixes if they're different from the one you selected as default with the USE command. As you've written it, you're not actually using the databases you created with CREATE DATABASE.