列不存在？

Question

想知道如果有人可以帮助我有点与此查询：

SELECT u1.id,count(DISTINCT u2.userstatus) as TEMPCOLUMN FROM users AS u1 
JOIN friendssym ON u1.id = friendssym.user_id 
JOIN (SELECT * FROM users) as u2 ON friendssym.friend_id=u2.id 
WHERE TEMPCOLUMN=1 
group by u1.id; 

我只想有个结果，其中计数（这是改名）等于1。我得到这个查询时出现错误：

ERROR: column "tempcolumn" does not exist 

但列应该存在的，对不对？任何人都可以协助谢谢！

Answer 1

您不能在WHERE子句中引用列别名。

SELECT u1.id, 
     COUNT(DISTINCT u2.userstatus) as TEMPCOLUMN 
    FROM USERS AS u1 
    JOIN friendssym ON u1.id = friendssym.user_id 
    JOIN USERS as u2 ON friendssym.friend_id = u2.id  
GROUP BY u1.id 
    HAVING COUNT(DISTINCT u2.userstatus) = 1 

在传统SQL中，最早可以引用列别名的是ORDER BY子句。但是MySQL和SQL Server允许访问HAVING和GROUP BY子句。