sql - Creating MySQL relationships with foreign keys and alter statements -

i try specific because feel understanding of subject isn't quite precise well. problem understanding how create relations between tables foreign keys add alter statements. 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 have make there relationships:

tag has 1 one connection category category has many 1 connection user user has 1 many connection article 

and 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 third 1 fails. when switch places of article , user constraint passes. after bit of digging , experimenting found out can't constraint 2 keys 1 either unique or primary key , other 1 not null. far don't know how continue, can please enlighten me how create 1 one, many one, 1 many , many many relationship between these tables because kinda lost , in head 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); 

first structure has errors: missing pk, why using int(30)? etc... 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 fks should 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);