Sql 如何联接返回一个结果的子查询？

我需要关于子查询的帮助。。 只是为了简化问题

我有一个客户表（只有唯一的客户）

我有一个销售表（有多个客户条目）

我想退回所有客户，但只显示他们最近购买的商品

John Smith     02/05/2018
Jane Smith     02/10/2018

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我想有人使用连接…

您可以使用

外部应用（Oracle 12c）

：

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或者：

WITH cte AS (
  SELECT c.*,s.Amount, s.DatePurchase, s.SaleId
     ROW_NUMBER() OVER(PARTITION BY c.Cust_id ORDER BY s.DatePurchase DESC) AS rn
  FROM Cust c
  LEFT JOIN Sales s
    ON c.Cust_id = s.Cust_id
)
SELECT *
FROM cte
WHERE rn = 1;

---

我会在这里使用

行号

：

SELECT CustID, CustName, SaleID, DatePurchase, Amount
FROM
(
    SELECT c.*, s.SaleID, s.DatePurchase, s.Amount,
        ROW_NUMBER() OVER (PARTITION BY c.CustID ORDER BY s.DatePurchase DESC) rn
    FROM customer c
    LEFT JOIN sales s
        ON c.CustID = s.CustID
) t
WHERE t.rn = 1;

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请注意，那些根本没有销售记录的客户将报告销售ID、购买日期和金额的

NULL

。如果需要，我们可以使用

COALESCE

用占位符替换这些占位符。

除了其他答案之外，还有一个选择是使用

密集排列

构造

因此，在你的情况下：

select c.CustID, max(s.DatePurchase) keep (dense_rank FIRST order by s.DatePurchase desc) 
from customers c
LEFT JOIN sales s on s.CustID = c.CustID
group by c.CustID;

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您还可以使用下面的简单子查询

SELECT c.Cust_Name,
    s.DatePurchase
FROM customer c
INNER JOIN sales s
INNER JOIN (SELECT Cust_ID, 
            MAX(DatePurchase) dpur 
            FROM sales GROUP BY Cust_ID) s1 ON s1.Cust_ID = s.Cust_ID
    AND s1.dpur = s.DatePurchase

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如果您只需要最新的购买日期，请使用

加入

和

分组方式

：

SELECT c.CustName, MAX(s.DatePurchase)
FROM customer c INNER JOIN
     sales s
     ON c.custid = s.custid
GROUP BY c.custid, c.custname;

select c.CustID, max(s.DatePurchase) keep (dense_rank FIRST order by s.DatePurchase desc) 
from customers c
LEFT JOIN sales s on s.CustID = c.CustID
group by c.CustID;

SELECT c.Cust_Name,
    s.DatePurchase
FROM customer c
INNER JOIN sales s
INNER JOIN (SELECT Cust_ID, 
            MAX(DatePurchase) dpur 
            FROM sales GROUP BY Cust_ID) s1 ON s1.Cust_ID = s.Cust_ID
    AND s1.dpur = s.DatePurchase

SELECT c.CustName, MAX(s.DatePurchase)
FROM customer c INNER JOIN
     sales s
     ON c.custid = s.custid
GROUP BY c.custid, c.custname;