Primary Key

[Caroline Jen]

I saw an example of creating tables (see below).  I
wonder what the primary key (user_name, role_name) in
the table user_roles means?  Does it mean that both
user_name and role_name are the primary key of the
user_roles table?  How does a table have two primary
keys?

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

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

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