选择计数表新字段中的不同列

Question

我有一个查询结果，有3个不同的p_id（3160、3301、41）

我想选择count（distinct p_id）并如图p_id_count_true（见图）所示，p_id_count不正确， 如何编写子查询并获取p_id_count_true

Answer 1

您似乎想要：

select . . .,
       count(distinct pid) over () as num_pids
from t

A，Postgres在窗口函数中不支持distinct。一种解决方法是：

select . . .,
       sum( (seqnum_pid = 1)::int ) over () as num_pids
from (select t.*, row_number() over (partition by pid order by pid) as seqnum_pid
      from t
     ) t

有些人更喜欢使用dense_rank()方法：

select . . .,
       max( seqnum_pid ) over () as num_pids
from (select t.*, dense_rank() over (order by pid) as seqnum_pid
      from t
     ) t