Case aggregate within aggregate
I'm struggling with a beast mode. I'd like to know the count of customers that placed an order multiple times (repeat customers). The only way I can think of is the following formula:
SUM( (CASE when COUNT(distinct `order_id`)>1 then 1 else 0 end) )
But I know that it is not possible because there already is an aggregate in the CASE-statement. And I'm not able to group it by customer_id. Can somebody help? Or do I need to create some kind of flag in the dataset?