具有左连接条件的MySQL case语句

Question

我在下面有一个MySQL请求，但它无法正常工作。

错误：

  

＃1054 - 未知专栏＆＃39; uid＆＃39;在＆＃39; on条款＆＃39;

查询：

SELECT 
  u.email,
  CASE 
  WHEN c.from_user_id = '301' THEN c.to_user_id 
  WHEN c.to_user_id = '301' THEN c.from_user_id 
  END AS uid
FROM `contacts` AS c
LEFT JOIN `users` AS u 
ON u.id = uid
HAVING uid >0

如何撰写正确的请求？

Answer 1

您不能在同一WHERE的{​​{1}}子句中引用列别名，因为SELECT用于在WHERE子句运行之前查找行在他们。把它放到子查询中。

SELECT