mysql - Creating a relationship between tables and creating a foreign key -

how make relation between these tables?

create table `categories` (   `id` smallint(6) not null,   `name` varchar(256) not null,   `description` text not null,   `position` smallint(6) not null,   primary key  (`id`) ) engine=innodb;  create table `pm` (   `id` bigint(20) not null,   `id2` int(11) not null,   `title` varchar(256) not null,   `user1` bigint(20) not null,   `user2` bigint(20) not null,   `message` text not null,   `timestamp` int(10) not null,   `user1read` varchar(3) not null,   `user2read` varchar(3) not null ) engine=innodb;  create table `topics` (   `parent` smallint(6) not null,   `id` int(11) not null,   `id2` int(11) not null,   `title` varchar(256) not null,   `message` longtext not null,   `authorid` int(11) not null,   `timestamp` int(11) not null,   `timestamp2` int(11) not null,   primary key  (`id`,`id2`) ) engine=innodb;  create table `users` (   `id` bigint(20) not null,   `username` varchar(255) not null,   `password` varchar(255) not null,   `email` varchar(255) not null,   `avatar` text not null,   `signup_date` int(10) not null ) engine=innodb; 

if topics has users author,

if storing id value of users author of topics in authorid column,

the "author" relationship one-to-many relationship. topics related 1 (or zero) users. (and users can "author" of zero, 1 or more topics.)

here's example of foreign key definition have database (innodb storage engine) enforce integrity of relationship:

alter table topics    add constraint fk_topics_users_author   foreign key (authorid) references users (id)   on update cascade on delete restrict