为什么我在JOIN中获得不明确的字段名“x”

事实1-我在BigQuery Web UI中成功运行了如下简单查询

SELECT
  a.id AS id,
  b.v AS v,
  c.w AS w
FROM (SELECT 1 AS id, "abc" AS x, "xyz" AS y) AS a
LEFT JOIN (SELECT "abc" AS x, "111" AS v) AS b ON a.x = b.x
LEFT JOIN (SELECT "xyz" AS y, "222" AS w) AS c ON a.y = c.y  

失败!

事实3-使用以下选项对表进行相同查询- 成功 !

问题 -为什么会出现此错误?

笔记 :
我在SO之外被问到这个问题,所以决定在这里分享。
正如您所看到的,“找到了解决方法”-因此,如果没有对实际问题进行解释/回答,请不要建议如何重新编写查询。