SQL-ex.ru #26 从两个表中选择平均值

Question

我在网站 http://www.sql-ex.ru/ 上遇到了有关 sql 查询的问题。查询要求 :

定义制造商 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)

我将查询写为：

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

问题是它没有返回正确的答案。平均回报率远高于预期。是不是平均数的计算方式有误？如何更改查询以使其返回预期答案？任何帮助将不胜感激。

谢谢

Answer 1

您分别计算个人电脑价格和笔记本电脑价格的平均值，然后将平均值合并在一起。您的查询很好，只是您不应该对子查询中的价格进行平均。只需返回子查询中的价格并在顶层求平均值即可：

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

Answer 2

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

对。

您查询的结果：

平均价格

754.1666

Answer 3

您的查询有误。这通常就像如何在分布式系统上聚合一样。您可以在分布式节点上进行聚合，然后带回聚合数据并在返回数据之上进行聚合，除非聚合是可转移的。 AVG 不可转让。 (1+2+3+4+5)/5 <> ((1+2)/2 + (1+2+3)/3)

Answer 4

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

Answer 5

也能用

SELECT Avg(price) AS avg_price
FROM   (SELECT pc.price
        FROM   pc
               INNER JOIN product
                       ON pc.model = product.model
        WHERE  maker = 'A'
        UNION ALL
        SELECT laptop.price
        FROM   laptop
               INNER JOIN product
                       ON laptop.model = product.model
        WHERE  maker = 'A') Any_name 

Answer 6

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

Answer 7

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'

Answer 8

SELECT AVG(price) FROM 
(
     SELECT PC.price, Product.maker FROM PC JOIN Product ON(PC.model=Product.model)
     UNION ALL
     SELECT Laptop.price, Product.maker FROM Laptop JOIN Product ON (Laptop.model = Product.model)
) T 
GROUP BY T.maker HAVING T.maker = 'A'

它之所以有效，是因为首先您选择所有笔记本电脑和个人电脑价格，接下来您仅选择制造商=“A”的记录，最后一步是计算平均价格。

Answer 9

Select avg(price) from
       ((select price,model  from pc 
       union all
       select price,model from laptop) a inner join  product on product.model=a.model and product.maker='A' )

首先，我觉得如果我把两张桌子都

union

price

和

model

的话会更好，然后我们可以从制造商A那里找到

average(price)

。

Answer 10

我（就个人而言）总是对 JOINS 有很大的挣扎，所以尝试这个没有连接的解决方案。 使用 IN 运算符

with cte as (
    SELECT price, model, code 
    FROM pc 
    WHERE model 
    IN (SELECT model FROM product WHERE maker = 'A')
UNION
    SELECT price, model, code 
    FROM laptop 
    WHERE model 
    IN (SELECT model FROM product WHERE maker = 'A')
)

SELECT AVG(price) FROM cte

Answer 11

select avg(P.price) from 
(select pc.model as model,pc.price as price from PC pc 
union all 
select L.model as model,L.price as price from Laptop L ) P
where P.model in 
( select model from Product where maker = 'A' )

Answer 12

select avg(price) as avg_price from
(select price
from Product, PC
where Product.model = PC.model
and maker = 'A'
union all
select price
from Product, Laptop
where Product.model = Laptop.model
and maker='A') avg_price