Extract customer_id that have been active on certain platforms

Question

I would like to extract the following information:

1. Distinct Customer_ids that have been active on platform A and B
2. Distinct Customer_ids that have been active on neither platform A and B (ie customers who have been active on any other platform other than A and B)

Main table

date          customer_id  platform
2019-01-01    1            A
2019-01-01    1            B
2019-01-01    2            A
2019-01-01    2            B
2019-01-01    3            A
2019-01-01    4            A
2019-01-01    4            B
2019-01-01    5            C
2019-01-01    6            D

Output table 1

Date         customerID_active_both_platforms
2019-01-01      1
2019-01-01      2
2019-01-01      4

Output table 2

Date         customerID_active_neither_platforms
2019-01-01      5
2019-01-01      6

Answer 1

You may handle this using conditional aggregation. For the first requirement of customers with both platforms A and B:

SELECT
    Date,
    customer_id AS customerID_active_both_platforms
FROM yourTable
WHERE
    platform IN ('A', 'B')
GROUP BY
    Date,
    customer_id
HAVING
    MIN(platform) <> MAX(platform);

For the second requirement, customers having neither platform A nor B:

SELECT
    Date,
    customer_id AS customerID_active_neither_platforms
FROM yourTable
GROUP BY
    Date,
    customer_id
HAVING
    COUNT(CASE WHEN platform IN ('A', 'B') THEN 1 END) = 0;

Answer 2

If you want this information per customer , then use two levels of aggregation -- first by customer/date and then by customer:

select date,
       sum( is_a * is_b ) as num_ab,
       sum( (1 - is_a) * (1 - is-b) ) as num_not_ab
from (select customer, date,
             max(case when platform = 'A' then 1 else 0 end) as is_a,
             max(case when platform = 'B' then 1 else 0 end) as is_b,
             max(case when platform not in ('A', 'B') then 1 else 0 end) as is_not_ab
      from t
      group by customer, date
     ) t
from t
group by date;

Of course, you can filter with a where clause to get customers that only meet one or the other condition.