Creating tables for user authentication - room for improvements?

At the moment I'm developing a web application. For that, I need to create a database for user authentication.

I have something like the following in mind:

create table users
(
id SMALLINT UNSIGNED NOT NULL AUTO_INCREMENT,
username VARCHAR(100) NOT NULL,
password VARCHAR(60) NOT NULL,
PRIMARY_KEY(id),
UNIQUE(username)
);

create table roles
(
id SMALLINT UNSIGNED NOT NULL AUTO_INCREMENT,
role VARCHAR(100) NOT NULL,
PRIMARY_KEY(id),
UNIQUE(role)
);

create table user_roles
(
user_id SMALLINT UNSIGNED NOT 开发者_StackOverflow中文版NULL,
role_id SMALLINT UNSIGNED NOT NULL,
PRIMARY_KEY(user_id)
UNIQUE (user_id, role_id),
);

Passwords are fixed size but I use varchar because I've read somewhere that in a table where you have both char and varchar columns, the char columns get converted to varchar.

Also, would it be beneficial to use FOREIGN KEY CONSTRAINTS?

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1. Don't use SMALLINTs for IDs. Seriously. Why would anybody do that!? Use a larger integer type.
2. The primary key on user_roles isn't going to work. Make (user_id, role_id) the primary key and put separate (non-unique) indexes on the fields you will be querying on.
3. You could add foreign keys that link user_roles to the other tables. If you use ON DELETE CASCADE, it'll even delete the stale links if you ever delete a user or a role.