SQL 查询解决方案的有效总结

Question

得到了这个 SQL 查询练习

Find out makers who produce only the models of the same type, and the number of those models exceeds 1. Deduce: maker, type

Table 信息：

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)

我写的版本（先在我的jotta pad上刮了一下lolz）：

SELECT Product.maker, Product.type
FROM Product
WHERE Product.maker IN
(SELECT X.maker
FROM
(SELECT DISTINCT maker,type 
FROM Product
) AS X
GROUP BY X.maker
HAVING COUNT(X.type) = 1
)
GROUP BY Product.maker,Product.type
HAVING COUNT(Product.model) > 1

这给了我正确的结果。但是，我相信这可能不是唯一的解决方案（至少不是理想的解决方案）- 因为我不是 SQL 向导。

我将不胜感激任何对答案的解释，但如果我能更好地简化它，我会用新的查询语句更新问题。

Ta

更新自从最初的帖子我总结为：

SELECT maker, type
FROM Product
WHERE Product.maker IN
(
SELECT maker
FROM Product
GROUP BY maker
HAVING COUNT(DISTINCT type)=1
) 
GROUP BY maker, type
HAVING COUNT(product.model) > 1

Answer 1

可以简化为：

SELECT  maker ,
        MAX(type) AS type
FROM    product
GROUP BY maker
HAVING  COUNT(DISTINCT type) = 1
        AND COUNT(*) > 1

此处您按制造商对结果进行分组，并且您只需要那些不同类型计数为 1 且组中总行数大于 1 的组。

你可以这样写：

HAVING  COUNT(DISTINCT type) = 1
        AND COUNT(DISTINCT model) > 1

但是和It is assumed that model numbers in the Product table are unique for all the makers and product types一样。

SQL 查询解决方案的有效总结

An Efficient Summarisation of this SQL Query Solution

sql

select

aggregate-functions