MySQL 错误帮助：On 子句中的未知列答案

【问题标题】：MySQL Error Help: Unknown Column in On ClauseMySQL 错误帮助：On 子句中的未知列
【发布时间】：2016-06-14 20:51:43
【问题描述】：

我正在尝试组合一个从多个表中提取数据的查询，但我遇到了一个奇怪的错误：

Error Code: 1054. Unknown column 'esjp_layout.section_id' in 'on clause'

引用的表和列确实存在，并且拼写正确。这是我正在使用的查询。有什么想法有什么问题吗？

SELECT
    esjp_section_refs.section_label,
    esjp_content.primary_key, esjp_content.content, esjp_content.summary_id, 
    esjp_role_refs.role_label, 
    esjp_users.first_name, esjp_users.last_name,
    FROM_UNIXTIME(esjp_content.sys_time)
FROM esjp_content
    INNER JOIN esjp_section_refs ON esjp_layout.section_id = esjp_section_refs.primary_key
    INNER JOIN esjp_layout ON esjp_content.primary_key = esjp_layout.content_id 
    INNER JOIN esjp_role_refs ON esjp_content.role_ref = esjp_role_refs.primary_key
    INNER JOIN esjp_users ON esjp_content.author_id = esjp_users.primary_key
WHERE esjp_layout.primary_key = 1
ORDER BY esjp_layout.section_id ASC, esjp_layout.position ASC ;

附注我知道这个查询有些冗长，但它是以编程方式组装的，所以字符数与我无关。

【问题讨论】：

表“esjp_layout”中可能存在名为“section_id”的列。但肯定不是在您尝试与特定 ON 子句连接的表“esjp_content”或“esjp_section_refs”中。

标签： mysql syntax

【解决方案1】：

问题是在ON 子句中加入的表还没有在查询中。

改为：

SELECT
    esjp_section_refs.section_label,
    esjp_content.primary_key, esjp_content.content, esjp_content.summary_id, 
    esjp_role_refs.role_label, 
    esjp_users.first_name, esjp_users.last_name,
    FROM_UNIXTIME(esjp_content.sys_time)
FROM esjp_content
    INNER JOIN esjp_layout ON esjp_content.primary_key = esjp_layout.content_id 
    INNER JOIN esjp_section_refs ON esjp_layout.section_id = esjp_section_refs.primary_key    
    INNER JOIN esjp_role_refs ON esjp_content.role_ref = esjp_role_refs.primary_key
    INNER JOIN esjp_users ON esjp_content.author_id = esjp_users.primary_key
WHERE esjp_layout.primary_key = 1
ORDER BY esjp_layout.section_id ASC, esjp_layout.position ASC ;

换句话说，您必须通过将表连接到已在查询中的表来加入表。可以这么说，它们必须按顺序加入。

【讨论】：

【解决方案2】：

您在 on 关闭后加入 esjp_layout，如果可能，尝试切换它，以便当 on 子句出现时，表已经加入

   FROM_UNIXTIME(esjp_content.sys_time)
   FROM esjp_content
   INNER JOIN esjp_section_refs ON **esjp_layout.section_id** = esjp_section_refs.primary_key
   INNER JOIN **esjp_layout** ON esjp_content.primary_key = esjp_layout.content_id

【讨论】：

【解决方案3】：

只有在加入后才能使用表。所以切换加入顺序为

SELECT
    esjp_section_refs.section_label,
    esjp_content.primary_key, esjp_content.content, esjp_content.summary_id, 
    esjp_role_refs.role_label, 
    esjp_users.first_name, esjp_users.last_name,
    FROM_UNIXTIME(esjp_content.sys_time)
FROM esjp_content
    INNER JOIN esjp_layout ON esjp_content.primary_key = esjp_layout.content_id 
    INNER JOIN esjp_section_refs ON esjp_layout.section_id = esjp_section_refs.primary_key        
    INNER JOIN esjp_role_refs ON esjp_content.role_ref = esjp_role_refs.primary_key
    INNER JOIN esjp_users ON esjp_content.author_id = esjp_users.primary_key
WHERE esjp_layout.primary_key = 1
ORDER BY esjp_layout.section_id ASC, esjp_layout.position ASC ;

【讨论】：