add 1596

Author: fishercoder1534

README.md

@@ -1096,6 +1096,7 @@ _If you like this project, please leave me a star._ ★
 |1633|[Percentage of Users Attended a Contest](https://leetcode.com/problems/percentage-of-users-attended-a-contest/)|[Solution](../master/database/_1633.sql) || Easy |
 |1623|[All Valid Triplets That Can Represent a Country](https://leetcode.com/problems/all-valid-triplets-that-can-represent-a-country/)|[Solution](../master/database/_1623.sql) || Easy |
 |1607|[Sellers With No Sales](https://leetcode.com/problems/sellers-with-no-sales/)|[Solution](../master/database/_1607.sql) || Easy |
+|1596|[The Most Frequently Ordered Products for Each Customer](https://leetcode.com/problems/the-most-frequently-ordered-products-for-each-customer/)|[Solution](../master/database/_1596.sql) || Medium |
 |1571|[Warehouse Manager](https://leetcode.com/problems/warehouse-manager/)|[Solution](../master/database/_1571.sql) || Easy |
 |1587|[Bank Account Summary II](https://leetcode.com/problems/bank-account-summary-ii/)|[Solution](../master/database/_1587.sql) || Easy |
 |1581|[Customer Who Visited but Did Not Make Any Transactions](https://leetcode.com/problems/customer-who-visited-but-did-not-make-any-transactions/)|[Solution](../master/database/_1581.sql) || Easy |

database/_1596.sql

@@ -0,0 +1,13 @@
+--credit: https://leetcode.com/problems/the-most-frequently-ordered-products-for-each-customer/discuss/861257/simple-and-easy-solution-using-window-function
+
+select customer_id, product_id, product_name from
+(
+    select o.customer_id, o.product_id, p.product_name,
+    rank() over (partition by customer_id order by count(o.product_id) desc) as ranking
+    from Orders o
+    join Products p
+    on o.product_id = p.product_id
+    group by customer_id, product_id
+) tmp
+where ranking = 1
+order by customer_id, product_id