SQL-ex.ru＃26从两个表中选择平均值

我对网站http://www.sql-ex.ru/上的sql查询有疑问。查询请求：SQL-ex.ru＃26从两个表中选择平均值

定义由制造商A.生产的PC和笔记本电脑的平均价格

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)

我写我的查询作为：

SELECT AVG(t.k) AS Avg_Price 
FROM 
    (SELECT AVG(A.price) AS k 
    FROM PC A 
    JOIN Product B ON(A.model=B.model) 
    WHERE B.maker='A' 
    UNION ALL 
    SELECT AVG(C.price) AS k 
    FROM Laptop C 
    JOIN PRODUCT D ON(C.model=D.model) 
    WHERE D.maker='A') AS t

问题是，它不retu正确的答案。返回的平均数远高于预期。平均值的计算方式是否错误？如何更改查询以便返回预期的答案？任何帮助，将不胜感激。

谢谢

来源

2013-03-27 Paul

您正在分别计算pc价格和笔记本电脑价格，将平均值平均在一起。您的查询很好，除非您不应该在子查询中对价格进行平均。只是在顶层返回子查询和平均价格：

select 
    AVG(Price) Avg_Price 
from 
(
    (
     select 
      pc.Price 
     from 
      PC pc 
      join Produt prod 
      on pc.Model = prod.Model 
     where 
      prod.Maker = 'A' 
    ) 
    union all 
    (
     select 
      pc.Price 
     from 
      Laptop l 
      join Produt prod 
      on l.Model = prod.Model 
     where 
      prod.Maker = 'A' 
    ) 
) q

来源

2013-03-27 06:10:03 Moho

啊平均是不可转让的古纳说。有意义 – Paul 2013-03-27 06:34:05

您的查询是错误的。通常就像你如何在分布式系统上进行聚合。您可以在分布式节点上进行聚合，然后取回聚合数据，并在返回数据之前进行聚合，只有聚合可以转移。 AVG不可转义。（1 + 2 + 3 + 4 + 5）/（1 + 2）/ 2 +（1 + 2 + 3）/ 3）

来源

2013-03-27 06:09:14

SELECT AVG(resultList.Price) 
FROM 
    (
     SELECT a.Price 
     FROM PC a INNER JOIN Product B 
        on a.Model = B.Model 
     WHERE B.Maker = 'A' 
     UNION ALL 
     SELECT c.Price 
     FROM Laptop c INNER JOIN Product d 
        on c.Model = d.Model 
     WHERE d.Maker = 'A' 
    ) resultList

来源

2013-03-27 06:13:21

SELECT AVG(datatable.price) FROM 
(
(SELECT PC.Price FROM PC INNER JOIN Product p1 ON PC.model=P1.model  
    WHERE P1.maker='A') 
UNION ALL 
(SELECT Laptop.price FROM Laptop INNER JOIN Product p2 ON 
    Laptop.model=P2.model WHERE P2.maker='A') 
) datatable

权。

你查询的结果：

Avg_Price

754.1666

来源

2014-02-20 08:13:43 learning

select AVG(P.price) as avg_price 
from 
(
select price, model from PC 
UNION ALL 
select price, model from Laptop 
) P 
join Product ON P.model = Product.model 
where maker = 'A'

来源

2015-10-08 09:34:08

如果你在这里添加一些解释会更好！ – 2015-10-08 09:42:31

with t1 as 
(SELECT price, type from pc left join product on product.model = pc.model where maker = 'A' 
UNION ALL 
SELECT price, type from laptop left join product on product.model = laptop.model where maker = 'A') 
select avg(price) from t1

来源

2016-10-14 10:09:01

SQL-ex.ru＃26从两个表中选择平均值

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