postgresql - 如何在 Postgres 中获得时间间隔的中位数？

Question

考虑一下：

SELECT
  review.clicker_id,
  sum(review.done - review.due) as timespent,
  PERCENTILE_DISC(0.5) WITHIN GROUP(ORDER BY timespent ASC) as median
FROM review WHERE monologue_id=7142 GROUP BY clicker_id ORDER BY timespent ASC;

我正在尝试获取（时间间隔）的中值sum(review.done - review.due)。但是，显然，我不允许这样做：

查询错误：错误：列“timespent”不存在第 4 行：
PERCENTILE_DISC(0.5) WITHIN GROUP(ORDER BY time ASC) as m...

我该怎么做？