Upsell Transactions

'''
We’re given a table of product purchases. Each row in the table represents an individual user product purchase.

Write a query to get the number of customers that were upsold, or in other words, the number of users who bought additional products after their first purchase.

Note: If the customer purchased two things on the same day, that does not count as an upsell, as they were purchased within a similar timeframe.
'''


select 

count(distinct t1.user_id) as num_of_upsold_customers
from transactions t1 inner join transactions t2 on t1.user_id = t2.user_id

where t1.created_at != t2.created_at

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select count(distinct user_id) as num_of_upsold_customers from (
select user_id, dense_rank() over (partition by user_id order by created_at) as rnk from transactions) as t1
where t1.rnk > 1

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SELECT COUNT(*) AS num_of_upsold_customers
FROM (
	SELECT user_id
	FROM transactions
	GROUP BY user_id
	HAVING COUNT(DISTINCT DATE(created_at)) > 1
) as t