SQL中有集合除法吗?

http://vadimtropashko.files.wordpress.com/2007/02/ch3.pdf

第32页:

关系除法不是一个基本的算子。它可以用投影、笛卡尔积和集差来表示。

Here is a nice explanation using relational algebra syntax

给定表格 sailors , boats reserves (来自罗摩克里希南和盖尔克的“数据库管理系统”的示例)您可以通过以下查询计算预订所有船只的海员:

SELECT name FROM sailors
WHERE Sid NOT IN (
    -- A sailor is disqualified if by attaching a boat,
    -- we obtain a tuple <sailor, boat> 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)
);

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