create domain positive_number 
    as numeric(12,5) not null 
	check (value > 0);
	
create table orders
(
  order_id     integer not null primary key,
  customer_id  integer not null references customers,
  amount       positive_number
);

create table employee
(
  employee_id    integer not null references customers,
  lastname       varchar(100) not null,
  firstname      varchar(100),
  salary         positive_number
);

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