create table users
(
  user_name varchar(15) not null primary key,
  user_pass varchar(15) not null
);

create table roles
(
  role_name varchar(15) not null primary key
);

create table user_roles
(
  user_name varchar(15) not null,
  role_name varchar(15) not null,
  primary key( user_name, role_name )
);