Question

我正在使用子查询，我需要在customers_notes中显示LEFT JOIN表格中的最新音符。我试过的查询是返回错误。我在下面做错了什么？

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 clause'中的未知列'n.customerid'

Answer 1

首先，您错过customerid作为您在子查询中选择的字段 - 这就是您收到错误的原因。

其次，即使你添加了它，你也不会得到你想要的结果。

我认为您有一个客户，一个客户可以附加多个注释，并且您希望获取客户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：

在子查询中查找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n.customerid条款中使用ON，您必须首先SELECT：

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。在左连接查询列名称中缺少customerid。

请参阅以下查询

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)

从LEFT JOIN子查询中选择Last Record

3 个答案: