Question

我有三张桌子：

Product(maker, model, type)
PC(model, speed, ram, hd, price)
Laptop(model, speed, ram, hd, screen, price)

我正在努力寻找销售笔记本电脑而不是电脑的制造商。我想我已经通过以下命令实现了这一目标：

select product.maker
from product
where product.maker not in 
    (select maker
    from pc, product
    where product.model = pc.model)
and product.maker in 
    (select maker
    from laptop, product
    where product.model = laptop.model);

但是我想在不使用IN和NOT IN的情况下重写它。

Answer 1

如果您不想使用not exists和not in，可以使用left join ... is null

select distinct p.maker
from product p
join laptop l on l.model = p.model
left join pc on pc.model = p.model
where pc.model is null

Answer 2

您应该可以使用连接重写此内容：

select distinct p.maker
from product p
left outer join product p1 on p.maker=p1.maker
    inner join pc on p1.model = pc.model 
inner join product p2 on p.maker=p2.maker
    inner join laptop on p2.model = laptop.model
where pc.model is null

product在您的查询中使用了三次;这个查询做同样的事情。但是，由于它使用没有子查询的ANSI连接，product与别名一起使用。

Answer 3

如果您想使用join执行此操作，我建议采用以下方法：

select p.maker
from product p left outer join
     pc
     on p.model = pc.model left outer join
     laptop l
     on p.model = l.model
group by p.maker
having sum(case when pc.model is not null then 1 else 0 end) = 0 and
       sum(case when l.model is not null then 1 else 0 end) > 0;

having子句可以等效地表示为：

having max(pc.model) is null and
       max(l.model) is not null;

使用not exists重写此内容仍然需要在子查询中加入。我怀疑type字段可能包含pc和laptop等信息。如果是这样，那么最简单的方法是：

select p.maker
from product p
group by p.maker
having sum(case when p.type = 'pc' then 1 else 0 end) = 0 and
       sum(case when p.type = 'laptop' then 1 else 0 end) > 0;

或者，或者：

select distinct pl.maker
from product pl left join
     product pp
     on pp.maker = pl.maker and
        pp.type = 'pc'
where pl.type = 'laptop' and pp.maker is null;

重写使用IN的sql命令

3 个答案: