1709-biggest-window-between-visits.sql

# Write your MySQL query statement below
SELECT user_id, MAX(diff) AS biggest_window
FROM
(
	SELECT user_id, 
	   DATEDIFF(COALESCE(LEAD(visit_date) OVER (PARTITION BY user_id ORDER BY visit_date), '2021-01-01'), visit_date) AS diff
	FROM userVisits
) a
GROUP BY user_id
ORDER BY user_id;