Como representar a divisão relacional (expressão de álgebra básica) em termos de SQL

create table Boats(
  bid int,
  bname varchar(50),
  color varchar(50)
);

create table Reserves(
  sid int,
  bid int,
  day date
);

-- All sailors who reserved at least one boat
SELECT DISTINCT sid
FROM Reserves

MINUS 

-- All sailors who reserved at least one boat, but not all of them
SELECT sid
FROM (
  -- all combinations of a sailor who reserved any boat with any boat
  -- available to be reserved:
  SELECT Reserves.sid, Boats.bid
  FROM
    Reserves
    CROSS JOIN
    Boats

  MINUS

  -- all combinations of sailor and boat for actual reservations made
  SELECT sid, bid
  FROM Reserves
) sids