MySQL相关子查询：子查询无法从外部查询中找到表？

Question

自从我使用相关子查询以来已经有一段时间了，我不确定我这样做是否正确。在我的子查询第二行最后一行中，我试图从外部表中获取node.id。当我尝试执行查询时，我得到了

错误代码：1054 未知列 'where 子句'中的'node.id'）

select node.id, node.title, depthLookup.depth
from posts node, (
    select count(parent.title) as depth
    from posts parent, posts children
    where children.lft > parent.lft 
    and children.rgt < parent.rgt
    and children.id = node.id
    order by parent.lft
) as depthLookup;

Answer 1

看来您只需要将表达式从子句 'from' 移动到字段列表

select node.id, node.title, 
(
    select count(parent.title) as depth
    from posts parent, posts children
    where children.lft > parent.lft 
    and children.rgt < parent.rgt
    and children.id = node.id
    order by parent.lft
) as depthLookup
from posts node;

或使用单值表，如：

select node.id, node.title, depthLookup.depth
from posts node,
(
    select count(parent.title) as depth
    from posts parent, posts children
    where children.lft > parent.lft 
    and children.rgt < parent.rgt
    and children.id = node.id
    order by parent.lft
) as depthLookup;