如何在 Postgres 中通过分区实现 Oracle 计数（不同）

Question

这是我在 select 语句中的 sql 示例。从 oracle 转换为 postgres，需要一种简单的方法来在 postgres 中重新实现 oracle count distinct over partition。

, count(distinct on pm.mobseg_state) over (partition by pm.trans_id) as mob_segments_count  
, count(distinct on pm.country_reg_region_cd) over (partition by pm.trans_id) as countries_count

Answer 1

Postgres 不直接支持count(distinct)。但是您可以使用子查询来实现它：

select . . .,
       sum( (seqnum_tm = 1)::int) as mob_segments_count ,
       sum( (seqnum_tr = 1)::int) as countries_count
from (select . . .,
             row_number() over (partition by pm.trans_id, pm.country_reg_region_cd order by pm.country_reg_region_cd) as seqnum_tr,
             row_number() over (partition by pm.trans_id, pm.mobseg_state order by pm.pm.mobseg_state) as seqnum_tm
      . . .
     ) . . .

这个想法很简单。在 partition by 键和不同的列上计算 row_number()。然后，只需将值为“1”时的次数相加。这需要一个子查询，因为您不能嵌套窗口函数。