add 1511

Author: fishercoder1534

README.md

@@ -1090,6 +1090,7 @@ _If you like this project, please leave me a star._ ★
 |1543|[Fix Product Name Format](https://leetcode.com/problems/fix-product-name-format/)|[Solution](../master/database/_1543.sql) || Easy |
 |1527|[Patients With a Condition](https://leetcode.com/problems/patients-with-a-condition/)|[Solution](../master/database/_1527.sql) || Easy |
 |1517|[Find Users With Valid E-Mails](https://leetcode.com/problems/find-users-with-valid-e-mails/)|[Solution](../master/database/_1517.sql) || Easy |
+|1511|[Customer Order Frequency](https://leetcode.com/problems/customer-order-frequency/)|[Solution](../master/database/_1511.sql) || Easy |
 |1495|[Friendly Movies Streamed Last Month](https://leetcode.com/problems/friendly-movies-streamed-last-month/)|[Solution](../master/database/_1495.sql) || Easy |
 |1484|[Group Sold Products By The Date](https://leetcode.com/problems/group-sold-products-by-the-date/)|[Solution](../master/database/_1484.sql) || Easy |
 |1407|[Top Travellers](https://leetcode.com/problems/top-travellers/)|[Solution](../master/database/_1407.sql) || Easy |

database/_1511.sql

@@ -0,0 +1,8 @@
+select customer_id, name from Customers
+join Orders using(customer_id)
+join Product using(product_id)
+group by customer_id
+having
+sum(if(left(order_date, 7) = '2020-06', quantity, 0) * price) >= 100
+and
+sum(if(left(order_date, 7) = '2020-07', quantity, 0) * price) >= 100;