Count Distinct with PostgreSQL

Due to the use-case it seems better to split the code.

Create a table month_brand based on the following query:

select      to_char(transaction_date, 'YYYY-MM')    as yyyymm
           ,brand
           ,count (distinct unique_mem_id)          as count_distinct_unique_mem_id

from        source.table

group by    yyyymm
           ,brand
;

Join month_brand with your source table:

select      t.transaction_date, t.brand, t.Description, t.Amount, t.Amount / m.count_distinct_unique_mem_id as newval

from                    source.table    as t

            join        month_brand     as m

            on          m.yyyymm = to_char(t.transaction_date, 'YYYY-MM')    

where       t.description ilike '%Criteria%'
;

Instead of count(distinct ...), 2 phase solution:

1. give row numbers to the duplicated unique_mem_id
2. count only the unique_mem_id with row_number = 1

select  *

into     temp.table

from   (SELECT  transaction_date, brand, Description, Amount, 
                (Amount / count(case rn when 1 then unique_mem_id end) over (partition by to_char(transaction_date, 'YYYY-MM'),brand)) as newval

        FROM    (SELECT     transaction_date, brand, Description, Amount,unique_mem_id
                            row_numner () over (partition by to_char(transaction_date, 'YYYY-MM'),brand,unique_mem_id) as rn

                 FROM       source.table
                 )
        )

WHERE   DESCRIPTION iLIKE '%Criteria%'
;