Question

我试图将一个子查询添加到select中，如下所示：

select ProductID,
       ProductName,
       concat((UnitsInStock / (select sum(UnitsInStock) from products))*100, '%')
       as Percent_of_total_units_in_stock
from products
order by ProductID;

/*
This query returns the same result as query above.
Here 3119 is hardcoded whereas query above uses
subquery to calculate 3119 on the fly.
*/
select ProductID,
       ProductName,
       concat((UnitsInStock / 3119)*100, '%')
       as Percent_of_total_units_in_stock
from products
order by ProductID;

对于以＆＃39; - ＆＃39;开头的3行，查询运行正常。注意：这只是为了表明问题 - 不会将其包含在实时查询中。

我含糊地理解我试图使用左连接运行内部查询，但这并不起作用。如果能够更好地解释这一点，那就太好了。

问题：有没有办法构建这个查询，所以我可以一起运行2？我尝试了几种配置，但我总是得到MySQL错误：

Unknown column 'a.receiver' in 'on clause'
Error code 1054.

感谢

Answer 1

我认为你在子查询中引用了错误的别名。注释代码说明：

FROM messages AS aa
JOIN users AS bb ON aa.sender=bb.userid
LEFT JOIN blocked AS cc ON cc.requester=aa.sender AND cc.receiver = a.receiver
LEFT JOIN blocked AS dd ON dd.receiver=aa.receiver AND dd.requester = a.receiver

子查询中的messages表具有别名aa。您在out查询中使用a作为别名。所以我认为这是一个错误，应该是这样的：

FROM messages AS aa
JOIN users AS bb ON aa.sender=bb.userid
LEFT JOIN blocked AS cc ON cc.requester=aa.sender AND cc.receiver = aa.receiver
LEFT JOIN blocked AS dd ON dd.receiver=aa.receiver AND dd.requester = aa.receiver

MySQL结合2个查询 - 内部联接问题

1 个答案: