text: alter type party_type add value 'customer';
  
  create table customers (
    party_id   uuid       not null,
    party_type party_type,
    primary key (party_id, party_type),
    foreign key (party_id, party_type)
            references parties (party_id, party_type),
    check (party_type = 'customer')
  );
  
  ---
  drop table customers;
  delete from pg_enum using pg_type
   where pg_type.oid       = pg_enum.enumtypid
     and pg_type.typname   = 'party_type'
     and pg_enum.enumlabel = 'customer';
  