一个不起作用的SQL示例

Question

我写了这段代码

select maker.product , product.type
from product
join laptop on laptop.model=product.model
join pc on pc.model=product.model
join printer on printer.model=product.model 
WHERE (product.type= 'pc' AND 
(NOT EXISTS (SELECT maker 
FROM Product
WHERE product.type = 'laptop'
))and (NOT EXISTS (SELECT maker 
FROM Product
WHERE product.type = 'printer'  
))
or 
product.type= 'laptop' AND 
(NOT EXISTS (SELECT maker 
FROM Product
WHERE product.type= 'pc' 
))and (NOT EXISTS (SELECT maker 
FROM Product
WHERE product.type= 'printer'
))
or
type = 'printer' AND 
(NOT EXISTS (SELECT maker 
FROM Product
WHERE product.type= 'laptop' 
))and (NOT EXISTS (SELECT maker 
FROM Product
WHERE product.type= 'pc' 
)))
group by product.maker
having count(product.model) > 1

在练习14中的这个网站sql-ex.ru但是没有用。 问题是： 对于Product表，以表格形式接收结果集，其中包含列：maker，pc，laptop和printer。对于每个制造商，如果制造商具有相应类型的产品，则该表必须包含“是”，否则必须包含“否”。在第一种情况下（是），在括号中指定（不含空格）相应类型的可用不同模型的数量（即在PC，笔记本电脑和打印机表中）。

它的方案是

抱歉我说英语不好！ 请帮忙 ！ 谢谢。

Answer 1

这是一个更易读的代码版本（除非我犯了错误）：

select maker.product , product.type
from product
join laptop on laptop.model=product.model
join pc on pc.model=product.model
join printer on printer.model=product.model 
WHERE (
  product.type= 'pc' AND 
  (NOT EXISTS (SELECT maker FROM Product WHERE product.type = 'laptop')) and
  (NOT EXISTS (SELECT maker FROM Product WHERE product.type = 'printer'))
  or 
  product.type= 'laptop' AND 
  (NOT EXISTS (SELECT maker FROM Product WHERE product.type= 'pc')) and
  (NOT EXISTS (SELECT maker FROM Product WHERE product.type= 'printer'))
  or
  type = 'printer' AND 
  (NOT EXISTS (SELECT maker FROM Product WHERE product.type= 'laptop')) and
  (NOT EXISTS (SELECT maker FROM Product WHERE product.type= 'pc'))
)
group by product.maker
having count(product.model) > 1

基本上，看起来你需要在每个OR之间做括号。