一：上码

-- 查询至少生产三种不同速度PC的厂商-- 分析：1.联合pc和product表，字段为厂商和速度 表1
--      2.按厂商名字进行分组统计厂商的个数，筛选条件为个数大于3的 表2
--      3.将表2 做为子表筛选出 maker-- 1.
-- select maker,speed
--     from pc,product
--     where pc.model = product.model;-- -- 2.
-- select maker,count(maker)
--     from (select maker,speed
--                 from pc,product
--                 where pc.model = product.model) AS temp
--      group by maker having count(maker) >= 3;-- 3.select maker from (select maker,count(maker)from (select distinct speed,makerfrom pc,productwhere pc.model = product.model) AS tempgroup by maker having count(maker) >= 3) as temp2;