add 1661

Author: fishercoder1534

README.md

@@ -1077,6 +1077,7 @@ _If you like this project, please leave me a star._ ★
 |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 |
 |1667|[Fix Names in a Table](https://leetcode.com/problems/fix-names-in-a-table/)|[Solution](../master/database/_1667.sql) || Easy |
+|1661|[Average Time of Process per Machine](https://leetcode.com/problems/average-time-of-process-per-machine/)|[Solution](../master/database/_1661.sql) || Easy |
 |1607|[Sellers With No Sales](https://leetcode.com/problems/sellers-with-no-sales/)|[Solution](../master/database/_1607.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 |
 |1543|[Fix Product Name Format](https://leetcode.com/problems/fix-product-name-format/)|[Solution](../master/database/_1543.sql) || Easy |

database/_1661.sql

@@ -0,0 +1,3 @@
+select s.machine_id, round(avg(e.timestamp - s.timestamp), 3) as processing_time
+from Activity s join Activity e
+on s.machine_id = e.machine_id and s.process_id = e.process_id and s.activity_type = 'start' and e.activity_type = 'end' group by s.machine_id;