Question

我正在努力解决练习14. http://www.sql-ex.ru/。查询要求：

找出只生产相同型号的制造商的制造商这些模型的数量超过1. Deduce：maker，type

数据库架构如下：

Product(maker, model, type)
PC(code, model, speed, ram, hd, cd, price)
Laptop(code, model, speed, ram, hd, screen, price)
Printer(code, model, color, type, price)

我写了以下查询;

select distinct maker, type
from product
where maker in (
select product.maker
from product, ( select model, code
        from printer
        union
        select model, code
        from pc
        union
        select model, code
        from laptop
           ) as T
where product.model = T.model
group by product.maker
having count(T.code) > 1 and count(distinct product.type) = 1
)

这不是正确的答案。我在这里缺少什么？

Answer 1

尝试此查询...

SELECT DISTINCT maker, type 
FROM product 
WHERE maker IN 
   (SELECT DISTINCT maker 
    FROM product 
    GROUP BY maker HAVING COUNT(distinct type) = 1
    AND count(model) > 1)

Answer 2

Claudiu Haidu的答案是正确的，尽管答案更短：

SELECT maker,MIN(type) 
FROM product 
GROUP BY maker 
HAVING COUNT(distinct type) = 1 AND COUNT(model) >  1

Answer 3

尽量使用max(type)中的select尽可能做到最好。

select maker, max(type)
from product
group by maker, type
having count(distinct type)=1 and count(model)>1

在没有where的情况下工作，使用max，但最多只有1个结果，你可以使用相同效果的min

Answer 4

select PC.Model,PC.speed,PC.hd 
from PC 
inner join Product ON Product.model = pc.model
where (pc.price < 500)

Answer 5

我刚试过，这段代码有效：

Select maker, type
from Product
group by maker
having count(distinct type) =1
and count(distinct model) > 1

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5 个答案: