左边加入的最近记录

Question

想象一下,我在SqlServer中有以下3个表:

Customer (CustomerID, FirstName, LastName)
Address (AddressID, CustomerID, Line1, City, State)
Product (ProductID, CustomerID, Description)

客户可以拥有多个送货地址和多个产品.

我想要做的是列出最新地址记录确定州的每个州的客户数量.例如"每个州有多少客户上次收到产品？".因此,我对客户以前的任何地址记录都不感兴趣,只对最近的地址记录感兴趣(由AddressID确定).

State | Number of Customers
--------------------------
CA    | 32
GA    | 12
TX    | 0
OH    | 18

我通常会这样做:

SELECT a.State, count(c.CustomerID)
FROM Product p
INNER JOIN Customer c ON c.CustomerID = p.CustomerID
LEFT JOIN Address a ON a.CustomerID = c.CustomerID
WHERE p.ProductID = 101
GROUP BY a.State

但是,由于客户可能有多个地址,客户只会被计入最新地址记录的状态？

PS以上纯粹是一个示例场景,可以轻松解释我想要实现的连接,并不反映实际的系统设计.

Answer 1

试试这个:

SELECT a.State, count(c.CustomerID)
FROM Product p
INNER JOIN Customer c ON c.CustomerID = p.CustomerID
LEFT JOIN Address a ON a.CustomerID = c.CustomerID 
      AND a.AddressID = 
        (
           SELECT MAX(AddressID) 
           FROM Address z 
           WHERE z.CustomerID = a.CustomerID
        )
WHERE p.ProductID = 101
GROUP BY a.State