add 1709

Author: fishercoder1534

README.md

@@ -1077,6 +1077,7 @@ _If you like this project, please leave me a star._ ★
 
 |  #  |      Title     |   Solutions   | Video         | Difficulty    | Tag          
 |-----|----------------|---------------|---------------|---------------|-------------
+|1709|[Biggest Window Between Visits](https://leetcode.com/problems/biggest-window-between-visits/)|[Solution](../master/database/_1709.sql) || Medium |
 |1693|[Daily Leads and Partners](https://leetcode.com/problems/daily-leads-and-partners/)|[Solution](../master/database/_1693.sql) || Easy |
 |1683|[Invalid Tweets](https://leetcode.com/problems/invalid-tweets/)|[Solution](../master/database/_1683.sql) || Easy |
 |1677|[Product's Worth Over Invoices](https://leetcode.com/problems/products-worth-over-invoices/)|[Solution](../master/database/_1677.sql) || Easy |

database/_1709.sql

@@ -0,0 +1,6 @@
+--# Write your MySQL query statement below
+with diff_table as (
+select user_id, datediff(lead(visit_date, 1, '2021-1-1') over (partition by user_id order by visit_date), visit_date) as diff from UserVisits
+)
+
+select user_id, max(diff) as biggest_window from diff_table group by user_id order by user_id