如何在Postgres中通過分區實現Oracle計數（獨特）

Question

這是我在選擇語句中的sql示例。從oracle轉換爲postgres，需要一種簡單的方法來重新實現Oracle數據在postgres中的分區。

, 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)。但是你可以用子查詢來實現它：

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 
     . . . 
    ) . . . 

這個想法很簡單。根據partition by鍵和不同的列計算row_number()。然後，只需加上值爲「1」的次數。這需要一個子查詢，因爲你不能嵌套窗口函數。