Mysql -从嵌套子查询引用外部字段

Question

如何从嵌套子查询中引用外部字段？

我将用一个例子很快地解释清楚：

SELECT
x
, (SELECT t1.x) as x1
/*, (SELECT x FROM (SELECT t1.x) as t2) as x2*/
FROM
(SELECT 1 as x UNION SELECT 2 as x UNION SELECT 3 as x) as t1;

如果我取消注释注释的子查询，我会得到一个"Unknown table 't1‘in field list“错误，但是我需要引用一个来自两级嵌套子查询的变量的复杂计算(连接是不可能的)。