在mysql语句中使用"as"时出现未知列错误 [英] Getting unknown column error when using 'as' in mysql statement

问题描述

我有这个MySQL语句:

I have this MySQL statement:

SELECT a.id, a.`from member_id`, a.`to member_id`, IF(a.`from member_id`=1, a.`to member_id`, a.`from member_id`) as other_id, a.text, MAX(a.`date sent`) as `date sent`
FROM message a
JOIN members m on other_id=m.id
WHERE (a.`from member_id`=1 OR a.`to member_id`=1) AND a.active=1
GROUP BY other_id
ORDER BY other_id DESC, `date sent` DESC

但是我得到了错误:

#1054 - Unknown column 'other_id' in 'on clause' 

我正在使用as键创建该列.有人知道这是怎么回事吗?

I am creating that column using the as key. Does anyone know whats wrong here?

谢谢.

推荐答案

as创建一个列别名(在本例中为other_id)，并且您无法加入列别名.您可以在ORDER BY中使用别名，但不能在其他地方使用，除非别名来自子查询.

The as creates a column alias (in this case other_id), and you can't join on a column alias. You can use an alias in the ORDER BY but nowhere else, unless the alias comes from a subquery.

这里最好的选择是在联接中重复IF函数:

Your best option here would be to repeat the IF function in the join:

SELECT
  a.id,
  a.from member_id,
  a.to member_id,
  IF(a.from member_id=1, a.to member_id, a.from member_id) as other_id,
  a.text,
  MAX(a.date sent) as date sent
FROM message a
JOIN members m on IF(a.from member_id=1, a.to member_id, a.from member_id) = m.id
WHERE (a.from member_id=1 OR a.to member_id=1) AND a.active=1
GROUP BY other_id
ORDER BY other_id DESC, date sent DESC

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