1709. Biggest Window Between Visits.sql

WITH user_visits_1709 AS (
SELECT*
FROM user_visits_1709
UNION ALL
SELECT DISTINCT user_id,'2021-1-1'::DATE
FROM user_visits_1709
),
ranked_visits AS (
SELECT*,
 ROW_NUMBER() OVER (PARTITION BY user_id ORDER BY visit_date) AS rn
FROM user_visits_1709
),
user_windows AS(
SELECT r1.*,r2.visit_date,r2.visit_date-r1.visit_dateAS win
FROM ranked_visits r1
INNER JOIN ranked_visits r2 ONr1.user_id=r2.user_idANDr1.rn+1=r2.rn
)
SELECT user_id,MAX(win) AS biggest_window
FROM user_windows
GROUP BY user_id;