[Users] Question about one tables's layout

[Juan Carlos Castro y Castro]

Why UNIQUE KEY? Wouldn't PRIMARY KEY be better?

CREATE TABLE dbaliases (
  alias_username varchar(64) default '' NOT NULL,
  alias_domain varchar(128) default '' NOT NULL,
  username varchar(64) default '' NOT NULL,
  domain varchar(128) default '' NOT NULL,
  UNIQUE KEY alias_key (alias_username,alias_domain),
  INDEX alias_user (username, domain)
);