SQL-ex.ru＃26從兩個表中選擇平均值

我對網站http://www.sql-ex.ru/上的sql查詢有疑問。查詢請求：SQL-ex.ru＃26從兩個表中選擇平均值

定義由製造商A.生產的PC和筆記本電腦的平均價格

數據庫架構如下：

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)

我寫我的查詢作爲：

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

問題是，它不retu正確的答案。返回的平均數遠高於預期。平均值的計算方式是否錯誤？如何更改查詢以便返回預期的答案？任何幫助，將不勝感激。

謝謝

來源

2013-03-27 Paul

您正在分別計算pc價格和筆記本電腦價格，將平均值平均在一起。您的查詢很好，除非您不應該在子查詢中對價格進行平均。只是在頂層返回子查詢和平均價格：

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

來源

2013-03-27 06:10:03 Moho

啊平均是不可轉讓的古納說。有意義 – Paul 2013-03-27 06:34:05

您的查詢是錯誤的。通常就像你如何在分佈式系統上進行聚合。您可以在分佈式節點上進行聚合，然後取回聚合數據，並在返回數據之前進行聚合，只有聚合可以轉移。 AVG不可轉義。（1 + 2 + 3 + 4 + 5）/（1 + 2）/ 2 +（1 + 2 + 3）/ 3）

來源

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

來源

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

權。

你查詢的結果：

Avg_Price

754.1666

來源

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'

來源

2015-10-08 09:34:08

如果你在這裏添加一些解釋會更好！ – 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

來源

2016-10-14 10:09:01

SQL-ex.ru＃26從兩個表中選擇平均值

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