PostgreSQL - Unexpected NULLs Returned When Joining on Two Nested Queries
I'm performance testing and I tried several approaches but none seem to work... I'm relatively new to this, so bear with me. I'm working with PostgreSQL 14 and I'm working with an scenario where my join between two nested queries is returning unexpected NULL values in the result set. I have two tables, `users` and `orders`, and I'm trying to fetch user details along with their latest order. The nested queries are structured to fetch the latest order for each user based on the order timestamp. However, I noticed that some users are being returned with NULLs for the order details even though they have placed orders. Here's the SQL I am using: ```sql SELECT u.id, u.name, o.order_id, o.order_date FROM ( SELECT id, name FROM users ) u LEFT JOIN ( SELECT user_id, order_id, order_date FROM orders WHERE order_date = ( SELECT MAX(order_date) FROM orders o2 WHERE o2.user_id = orders.user_id ) ) o ON u.id = o.user_id; ``` While I expected to see all users with their latest orders, the result set has users with NULL for `order_id` and `order_date`. I tried changing the join to an INNER JOIN, but then users without orders get filtered out completely, which is not what I want. I also double-checked the data and confirmed that there are indeed users with orders that should match. To debug this, I ran the inner query independently and verified that it returns the expected results, but when combined with the outer query, it drops the expected rows. Could someone guide to understand why I'm getting these NULL values in my final result set? Is there a better way to structure this query to get the desired output without losing users who have no orders? My development environment is Linux. Any ideas what could be causing this? This is part of a larger API I'm building. Am I missing something obvious? I'm working with Sql in a Docker container on Windows 10. Could this be a known issue?