テーブルを結合した後の元の数を取得するには？

Question

私は各店舗の製品数/システム数を探しています。テーブルは次のようになります。

Store 
Id 
100 
200 

Customer 
Id  | dealerId 
1  | 100 
2  | 200 

System 
Id | CustomerId 
20 | 2 
30 | 2 
40 | 2 
50 | 1 

Product 
Id | SystemId 
1000 | 20 
2000 | 50 

は私が取得しようとしています：

storeId | Number of systems | number of products | average 
100  |   1  |   1   |  1/1 
200  |   3  |   1   |  1/3  

私はこのクエリを書かれています。正しい数の製品を手に入れることができますが、私はテーブルに参加しているのでシステムの台数が狂っています。各店舗のシステムの合計数を得る方法はありますか？

SELECT 
    s.Id as Store, 
    COUNT(Distinct SystemsIden) as NumOfSystems, 
    COUNT(distinct ProductIden) as NumOfProduct, 
    CAST(COUNT(distinct ProductIden)as float)/CAST(COUNT(Distinct SystemsIden) as 
    float) as average 
FROM 
Store s 
    INNER JOIN 
    (
    Select systemsiden,CustomerIden,ProductIden, Customer.StoreId as 
    storeiden from 
(
    select Product.ID as ProductIden, System.Id as systemsiden, System.customerId as CustomerIden from product join Ssystem On System.Id = Product.SystemIdId 
) 
    table1 join Customer 
    on Customer.Id = CustomerIden 
) 
    table2 On s.Id = storeiden 
GROUP BY 
    s.Id 

Answer 1

豊富なサブクエリを使用するための必要はありません、簡単な外部結合十分でしょう。

SELECT st.Id, 
     COUNT(DISTINCT sy.Id) AS SystemCount, 
     COUNT(DISTINCT pr.Id) AS ProductCount, 
     CASE WHEN COUNT(DISTINCT sy.Id) = 0 THEN 0 
      ELSE 1.0 * COUNT(DISTINCT pr.Id)/COUNT(DISTINCT sy.Id) 
     END AS Average 
FROM Store AS st 
LEFT JOIN Customer AS cu ON cu.DealerId = st.Id 
LEFT JOIN System AS sy ON sy.CustomerId = cu.Id 
LEFT JOIN Product AS pr ON pr.SystemId = sy.Id 
GROUP BY st.Id; 

Answer 2

SELECT 
    s.Id as Store, 
    COUNT(DISTINCT sys.id) as NumOfSystems, 
    COUNT(DISTINCT p.id) as NumOfProduct, 
    IFNULL(COUNT(DISTINCT p.id),0)/IFNULL(COUNT(Distinct sys.id),1) as average 
FROM Store s 
INNER JOIN Customer c 
ON c.dealerId = s.id 
INNER JOIN System sys 
ON sys.customerId = c.id 
INNER JOIN product p 
ON sys.id=p.systemId 
GROUP BY s.Id