sql - Postgres 9.4 的中位数 - 为什么我的代码不工作？

Question

SELECT 
(users.created_at::timestamp - invitations.created_at::timestamp) as elapsed,
percentile_disc(0.50)
      WITHIN GROUP (ORDER BY elapsed) as median
 FROM invitations
 join users on users.id = invitations.user_id

试图找出从创建邀请到用户接受邀请之间的平均等待时间。此代码会产生错误:

ERROR: column "elapsed" does not exist.

如何修复？

Answer 1

您不能在定义别名的同一级别上使用别名。您需要将其包装在派生表中:

select elapsed, 
       percentile_disc(0.50) WITHIN GROUP (ORDER BY elapsed) as median
from (
  SELECT users.created_at::timestamp - invitations.created_at::timestamp as elapsed,
  FROM invitations
    join users on users.id = invitations.user_id
) t;