feat: add solutions to lc problem: No.1709

No.1709.Biggest Window Between Visits

Author: yanglbme

solution/1700-1799/1709.Biggest Window Between Visits/README.md

@@ -68,7 +68,27 @@
 ### **SQL**
 
 ```sql
-
+# Write your MySQL query statement below
+select
+    user_id,
+    max(datediff(nxt_day, visit_date)) as biggest_window
+from
+    (
+        select
+            user_id,
+            visit_date,
+            lead(visit_date, 1, '2021-1-1') over (
+                partition by user_id
+                order by
+                    visit_date
+            ) as nxt_day
+        from
+            UserVisits
+    ) t
+group by
+    user_id
+order by
+    user_id;
 ```
 
 <!-- tabs:end -->

solution/1700-1799/1709.Biggest Window Between Visits/README_EN.md

@@ -71,7 +71,27 @@ For the third user, the only window in question is between dates 2020-11-11 and
 ### **SQL**
 
 ```sql
-
+# Write your MySQL query statement below
+select
+    user_id,
+    max(datediff(nxt_day, visit_date)) as biggest_window
+from
+    (
+        select
+            user_id,
+            visit_date,
+            lead(visit_date, 1, '2021-1-1') over (
+                partition by user_id
+                order by
+                    visit_date
+            ) as nxt_day
+        from
+            UserVisits
+    ) t
+group by
+    user_id
+order by
+    user_id;
 ```
 
 <!-- tabs:end -->

solution/1700-1799/1709.Biggest Window Between Visits/Solution.sql

@@ -0,0 +1,21 @@
+# Write your MySQL query statement below
+select
+    user_id,
+    max(datediff(nxt_day, visit_date)) as biggest_window
+from
+    (
+        select
+            user_id,
+            visit_date,
+            lead(visit_date, 1, '2021-1-1') over (
+                partition by user_id
+                order by
+                    visit_date
+            ) as nxt_day
+        from
+            UserVisits
+    ) t
+group by
+    user_id
+order by
+    user_id;