Creating MySQL relationships with foreign keys and alter statements - mysql

Question

I will try to be specific because I feel my understanding of the subject isn't quite precise as well. My problem is understanding how to create relations between tables with foreign keys which I add with alter statements. I have these tables.
CREATE TABLE article (
content TEXT NOT NULL,
published_on DATE NOT NULL,
created_on DATE NOT NULL,
);
CREATE TABLE category(
name VARCHAR(30) NOT NULL,
date_created DATE NOT NULL,
);
CREATE TABLE user(
income FLOAT(30, 30) NOT NULL,
password VARCHAR(30) NOT NULL,
picture_url TEXT NOT NULL,
);
CREATE TABLE tag(
description VARCHAR(30) NOT NULL,
second_priority FLOAT(30, 30) NOT NULL,
);
To which I have to make there relationships:
Tag has a one to one connection to Category
Category has a many to one connection to User
User has a one to many connection to Article
And to do that I use there statements:
ALTER TABLE tag ADD CONSTRAINT FOREIGN KEY (tag_id) REFERENCES category (category_id);
ALTER TABLE category ADD CONSTRAINT FOREIGN KEY (user_id) REFERENCES user (user_id);
ALTER TABLE user ADD CONSTRAINT FOREIGN KEY (user_id) REFERENCES article (user_id);
My problem is that the third one fails. When I switch the places of article and user the constraint passes. After a bit of digging and experimenting I found out that I can't constraint two keys from which one is either UNIQUE or PRIMARY KEY and the other one just NOT NULL. So far I don't know how to continue, can someone please enlighten me as to how to create a one to one, many to one, one to many and a many to many relationship between these tables because I am kinda lost and everything in my head is a mess.
FULL STUFF:
CREATE DATABASE exam_database;
USE exam_database;
CREATE TABLE article (
content TEXT NOT NULL,
published_on DATE NOT NULL,
created_on DATE NOT NULL,
user_id INT(30) NOT NULL
);
CREATE TABLE category(
name VARCHAR(30) NOT NULL,
date_created DATE NOT NULL,
category_id INT(30) NOT NULL PRIMARY KEY AUTO_INCREMENT UNIQUE,
user_id INT(30) NOT NULL
);
CREATE TABLE user(
income FLOAT(30, 30) NOT NULL,
password VARCHAR(30) NOT NULL,
picture_url TEXT NOT NULL,
user_id INT(30) NOT NULL PRIMARY KEY AUTO_INCREMENT UNIQUE
);
CREATE TABLE tag(
description VARCHAR(30) NOT NULL,
second_priority FLOAT(30, 30) NOT NULL,
tag_id INT(30) NOT NULL PRIMARY KEY AUTO_INCREMENT UNIQUE
);
ALTER TABLE tag ADD CONSTRAINT FOREIGN KEY (tag_id) REFERENCES category (category_id);
ALTER TABLE category ADD CONSTRAINT FOREIGN KEY (user_id) REFERENCES user (user_id);
ALTER TABLE user ADD CONSTRAINT FOREIGN KEY (user_id) REFERENCES article (user_id);

Answer 1

First Your structure has some errors:
Missing PK, Why using INT(30)? etc...
Your DB should looks like:
CREATE DATABASE exam_database;
USE exam_database;
CREATE TABLE article (
id INT NOT NULL PRIMARY KEY AUTO_INCREMENT UNIQUE,
user_id INT NOT NULL,
content TEXT NOT NULL,
published_on DATE NOT NULL,
created_on DATE NOT NULL
);
CREATE TABLE category(
id INT NOT NULL PRIMARY KEY AUTO_INCREMENT UNIQUE,
user_id INT NOT NULL,
name VARCHAR(30) NOT NULL,
date_created DATE NOT NULL
);
CREATE TABLE user (
id INT NOT NULL PRIMARY KEY AUTO_INCREMENT UNIQUE ,
income FLOAT(30, 30) NOT NULL,
password VARCHAR(30) NOT NULL,
picture_url TEXT NOT NULL
);
CREATE TABLE tag (
id INT NOT NULL PRIMARY KEY AUTO_INCREMENT UNIQUE,
category_id INT NOT NULL,
description VARCHAR(30) NOT NULL,
second_priority FLOAT(30, 30) NOT NULL
);
Second Your FKs should look like:
ALTER TABLE tag ADD CONSTRAINT FOREIGN KEY (category_id) REFERENCES category (id);
ALTER TABLE category ADD CONSTRAINT FOREIGN KEY (user_id) REFERENCES user (id);
ALTER TABLE article ADD CONSTRAINT FOREIGN KEY (user_id) REFERENCES user (id);