Here is one method:
select user_id from orders where created >= date_format(current_date, '%Y-%m-01') and created < date_format(current_date, '%Y-%m-01') + interval 1 month group by user_id having count(distinct date(created)) = day(date_format(current_date, '%Y-%m-01') + interval 1 month - interval 1 day)
How this works:
whereclause filters on the previous month
then we group by
havingclause counts how many distinct days appear in each group, and ensures that it is equal to the number of days in the month; to compute the number of days within last month, we use date arithmetics: basically this is equal to the day number of the last day of the month
Disclaimer: I read this as a MySQL question, but now I see that there is no such tag in your question. The logic would be the same in other databases, you would just need to adjust the date arithmetics logic.
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