Is there set division in SQL?

Question

I\'m fully aware that set division can be accomplished through a series of other operations, so my question is:

Is there a command for set division in SQL?

Answer 1

Here is a nice explanation using relational algebra syntax.

Given tables sailors, boats and reserves (examples from Ramakrishnan & Gehrke's "Database Management Systems") you can compute sailors who have reserved all boats with the following query:

SELECT name FROM sailors
WHERE Sid NOT IN (
    -- A sailor is disqualified if by attaching a boat,
    -- we obtain a tuple  that is not in reserves
    SELECT s.Sid
    FROM sailors s, boats b
    WHERE (s.Sid, b.Bid) NOT IN (
        SELECT Sid, Bid FROM reserves
    )
);

-- Alternatively:
SELECT name FROM sailors s
WHERE NOT EXISTS (
    -- Not reserved boats
    (SELECT bid FROM boats)
    EXCEPT
    (SELECT r.bid FROM reserves r
    WHERE r.sid = s.sid)
);