在嵌套的选择子查询中使用别名

Question

我有以下查询：

SELECT t.*,
    (SELECT SUM(subs)
    FROM (SELECT …
    FROM (SELECT prod_id, COUNT(*) subs
        FROM subscriptions s
        WHERE s.begin_date < t.week_of + interval 7 day AND
              s.end_date >= t.week_of
       GROUP BY 1) A
    LEFT JOIN …
    LEFT JOIN …
    WHERE …
    GROUP BY 1, 2) B) as num_subs
FROM (SELECT TIMESTAMPADD(week, TIMESTAMPDIFF(week, '2015-01-01', 
Ticket.timestamp), '2015-01-01') as week_of, COUNT(*) as Count_Tickets
      FROM . . .  -- rest of query here
     ) t ;

我正在尝试使用在from部分中创建的t.week_of列来执行s.begin_date和s.end_date操作，但是我一直遇到Unknown column 't.week_of' in 'where clause'错误。我的语法有什么问题？如何在WHERE s.begin_date < t.week_of...行中使用t别名？