I have a Clients table in PostgreSQL (version 9.1.11), and I would like to write a query to filter that table. The query should return only clients which meet one of the following conditions:
--The client's last order (based on orders.created_at) has a fulfill_by_date in the past.
OR
--The client has no orders at all
I've looked for around 2 months, on and off, for a solution.
I've looked at custom last aggregate functions in Postgres, but could not get them to work, and feel there must be a built-in way to do this.
I've also looked at Postgres last_value window functions, but most of the examples are of a single table, not of a query joining multiple tables.
Any help would be greatly appreciated! Here is a sample of what I am going for:
Clients table:
| client_id | client_name |
----------------------------
| 1 | FirstClient |
| 2 | SecondClient |
| 3 | ThirdClient |
Orders table:
| order_id | client_id | fulfill_by_date | created_at |
-------------------------------------------------------
| 1 | 1 | 3000-01-01 | 2013-01-01 |
| 2 | 1 | 1999-01-01 | 2013-01-02 |
| 3 | 2 | 1999-01-01 | 2013-01-01 |
| 4 | 2 | 3000-01-01 | 2013-01-02 |
Desired query result:
| client_id | client_name |
----------------------------
| 1 | FirstClient |
| 3 | ThirdClient |