从LEFT JOIN子查询中选择Last Record

Question

我正在玩子查询，我需要在LEFT JOIN显示一个customers_notes表中的最新笔记。 我尝试的查询返回错误。 我在下面做什么错了？

SELECT c.*, n.datesubmitted                    
FROM `customers` c 
LEFT JOIN (SELECT datesubmitted FROM `customers_notes` 
            ORDER BY datesubmitted DESC LIMIT 1) AS n           
                    ON (c.customerid = n.customerid)

我得到的错误是： ＃1054-'on子句'中的未知列'n.customerid'

Answer 1

首先，您缺少在子查询中选择的customerid作为字段-这就是为什么会出现错误的原因。

其次，即使您添加了它，也不会获得想要的结果。

我假设您有一个客户，一个客户可以附加多个注释，而您想获取该客户的详细信息以及他最近的注释的所有详细信息。

解决此类问题的一般方法是：

在子查询中找到max()日期，如下所示：

select customerid, max(datesubmitted)
  from customer_notes
  group by customerid

然后在外部查询中加入这些结果

select c.*, n.*
  from customers c
    left join customer_notes n
      on c.customer_id = n.customer_id
    left join (
      select customerid, max(datesubmitted)
        from customer_notes
        group by customerid ) nn
      on n.datesubmitted = nn.datesubmitted

Answer 2

要在ON子句中使用n.customerid ，必须首先SELECT它：

SELECT c.*, n.datesubmitted                    
FROM `customers` c 
LEFT JOIN (SELECT datesubmitted, customerid FROM `customers_notes` 
            ORDER BY datesubmitted DESC LIMIT 1) AS n           
                    ON (c.customerid = n.customerid)

Answer 3

您缺少要加入的必填列。 在左联接查询列名称customerid丢失。

请参考以下查询

SELECT c.*, n.datesubmitted                    
FROM `customers` c 
LEFT JOIN (SELECT  customerid,datesubmitted FROM `customers_notes` 
        ORDER BY datesubmitted DESC LIMIT 1) AS n           
                ON (c.customerid = n.customerid)