Why should I create a MySql table containing index only? - mysql

Question

I'm following a Java Spring tutorial to learn some basic information about the secure login in a web app.
In this tutorial, the author has created 3 MySql table to manage the authentication:
CREATE TABLE `roles` (
`id` int(6) NOT NULL AUTO_INCREMENT,
`role` varchar(20) NOT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=5 DEFAULT CHARSET=utf8;
CREATE TABLE `users` (
`id` int(6) NOT NULL AUTO_INCREMENT,
`login` varchar(20) NOT NULL,
`password` varchar(20) NOT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=5 DEFAULT CHARSET=utf8;
CREATE TABLE `user_roles` (
`user_id` int(6) NOT NULL,
`role_id` int(6) NOT NULL,
KEY `user` (`user_id`),
KEY `role` (`role_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
The table "roles" contains the user role (for example "Admin", "User" etc...).
The table "users" contains the user login and password.
So, I can't understand why the table "user_roles" has been created! The relation between "roles" and "users" is one-to-one, so I could insert an index for these 2 tables and delete the "user_roles"...is it right?
Why should I need to join the tables "users --> user_roles --> roles" instead of "users --> roles" ?
Thanks in advance :)

Answer 1

For a 1:1 relationship, have just one table. (There are exceptions, but I don't see any reason for such here.)
If you have many users in each role but a user is in only one role, then add role_id to the Users table. This is 1:many. You may need INDEX(role_id).
If a user can have many roles and each role can have many users, then you need many:many. And this would be the optimal way to write the 3rd table:
CREATE TABLE `user_roles` (
`user_id` int(6) NOT NULL,
`role_id` int(6) NOT NULL,
PRIMARY KEY (`user_id`, role_id),
KEY (`role_id`, user_id)
) ENGINE=InnoDB DEFAULT;
In some sense, that is an index-only table, since the PRIMARY KEY contains all the fields.
The (6) in INT(6) is meaningless. In particular, it does not give you 6-digit integers, it still gives you 4-byte signed integers up to 2 billion. Perhaps you should use MEDIUMINT UNSIGNED for values of 0..16M. Or SMALLINT UNSIGNED for 2-byte values of 0..64K.