Finding Active customers by month- SQL- Big Query

Question

I want to find a way to get the count of customers with promotion active per month, with the conditions on the dates included on the case statement. I tried the following query but there's an overlap in some of the ranges and I am not able not get the same count if i do one case per month. My idea was to create a loop/recursive method that takes the start and end time, but I'm having issues with the creation.

    SELECT
   region,
  #June data
  CASE
    WHEN DATE (start_time) >= '2022-06-01' AND DATE(end_time) <= '2022-06-30' 
    OR ( DATE(start_time) < '2022-06-01'AND DATE(end_time) >= '2022-06-01' AND DATE(end_time)<='2022-06-30')
     OR (DATE(end_time)>'2022-06-30' AND DATE(start_time)>='2022-06-01' AND DATE(start_time)<='2022-06-30') 
    THEN '2022-06-01'
  #July data
  WHEN DATE (start_time) >= '2022-07-01'AND DATE(end_time) <= '2022-07-31'
  OR ( DATE(start_time) < '2022-07-01'AND DATE(end_time) >= '2022-07-01'AND DATE(end_time)<='2022-07-31')
  OR (DATE(end_time)>'2022-07-31'AND DATE(start_time)>='2022-07-01'AND DATE(start_time)<='2022-07-31') 
  THEN '2022-07-01'
  #August data
    WHEN DATE (start_time) >= '2022-08-01' AND DATE(end_time) <= '2022-08-31' 
    OR ( DATE(start_time) < '2022-08-01'AND DATE(end_time) >= '2022-08-01' AND DATE(end_time)<='2022-08-31') 
    OR (DATE(end_time)>'2022-08-31' AND DATE(start_time)>='2022-08-01' AND DATE(start_time)<='2022-08-31')
     THEN '2022-08-01'
  ELSE
  STRING(DATE(DATE_TRUNC(start_time,month)))
END
  AS month,
  COUNT(DISTINCT customer_id) AS active_customers,
FROM
customers_table 
GROUP BY region,month

Let me know if more clarity is needed please, it is one of my first questions. Thanks!

Answer 1

I'm assuming you want each row in WHEN to be an 'OR'. So the first row needs to be also wrapped in parenthesis.

Also each statement may be shortened using DATE_TRUNC()

Here is the example for the first WHEN:

    WHEN 
     (DATE (start_time) >= '2022-06-01' AND DATE(end_time) <= '2022-06-30')
     OR ( DATE(start_time) < '2022-06-01'AND DATE_TRUNC(end_time,month) = '2022-06-01')
     OR (DATE(end_time)>'2022-06-30' AND DATE_TRUNC(start_time,month) ='2022-06-01') 
    THEN '2022-06-01'

Hope that helps!