一：上码

--  查询只卖三种不同型号PC的厂商--  分析：1.首先 pc和product表联合 字段为厂商
--        2.统计这些厂商的个数过滤掉型号小于3的厂商 表1
--        3.然后laptop和product联合 表2，printer和product联合 字段均为厂商 表3
--        4.将表1作为子表，查询条件为 不在表2和表3中 厂商-- 1.
-- select maker
--      from pc,product
--      where pc.model = product.model;-- -- 2.
-- select maker,count(maker) 
--        from (select maker
--                      from pc,product
--                      where pc.model = product.model) as temp
--        group by maker having count(maker) >= 3;-- -- 3.
-- select maker
--         from laptop,product
--         where laptop.model = product.model;-- select maker 
--         from printer,product
--         where product.model = printer.model;-- 4.      
select makerfrom (select maker,count(maker) from (select makerfrom pc,productwhere pc.model = product.model) as tempgroup by maker having count(maker) >= 3) as tempwhere temp.maker not in (select makerfrom laptop,productwhere laptop.model = product.model)and temp.maker  not in(select maker from printer,productwhere product.model = printer.model);