T-SQL solution for a question of "Find users for whom each booking is within 7 days of their most recent booking"

frequent_users.sql

-- https://medium.com/@mail2asimmanna/another-beautiful-sql-question-from-my-business-analyst-interview-1d9fa00c0381

DROP TABLE IF EXISTS #bookings;

SELECT 
	CAST(u AS smallint) AS userid,
	CAST(d AS date) AS booking_date
INTO #bookings
FROM 
(VALUES (1,'2024-01-01')
 ,(1,'2024-01-03')
 ,(1,'2024-01-09')
 ,(2,'2024-01-10')
 ,(2,'2024-01-18')
 ,(2,'2024-01-20')
 ,(3,'2024-01-10')
 ,(3,'2024-01-19')
 ,(3,'2024-01-30')
 ,(4,'2024-01-05')
 ,(4,'2024-01-09')
 ,(4,'2024-01-11')
 ,(4,'2024-01-19')) AS t(u, d);

--SELECT DISTINCT 
--	SQL_VARIANT_PROPERTY(
--		CONVERT(sql_variant, booking_date),
--		'BaseType'
--	) AS type_of_booking_date 
--FROM #bookings;

WITH dat 
AS (
	SELECT 
		userid, 
		booking_date, 
		DATEDIFF(
			DAY, 
			LAG(booking_date) OVER (PARTITION BY userid ORDER BY booking_date), 
			booking_date
		) AS diff
	FROM #bookings
)
SELECT userid AS frequent_user_userid
FROM dat 
EXCEPT
SELECT userid
FROM dat
WHERE diff > 7;