Extract subset from the main query in SQL

Question

In the below query, I get the count of customers who were active between "2017-09-01 00:00:00" and "2017-11-31 23:59:59" as cust_90 and would like to add another column to find the count of customers who were active between "2017-11-01 00:00:00" and "2017-11-31 23:59:59" (a subset of the whole period).

    select custid, count(distinct concat(visit1, visit2)) as cust_90
    from test1
    where date_time between "2017-09-01 00:00:00" and "2017-11-31 23:59:59"
    and custid = '234214124'
    group by custid;

Sample output:

    CustomerName    cust_90     cust_30
    David           38           15

Wondering whether I could have a subquery in the above query to find the customers active in a month. Any suggestions would be great.

Answer 1

This is called conditional aggregation which can be done using a case expression.

select custid, 
count(distinct concat(visit1, visit2) end) as cust_90,
count(distinct case when to_date(date_time)>='2017-11-01' then concat(visit1, visit2) end) as cust_30
from test1
where date_time >= '2017-09-01' and date_time < '2017-12-01'
and custid = '234214124'
group by custid;