PostgreSQL: advanced patterns with GROUP BY and Joins leading to inflated counts
I've been researching this but I'm running into an scenario with a PostgreSQL query where I expect to get a count of distinct users associated with orders, but the results seem inflated due to the way I'm joining tables and grouping the results. My current query looks something like this: ```sql SELECT o.order_id, COUNT(DISTINCT u.user_id) as user_count FROM orders o JOIN users u ON o.user_id = u.user_id GROUP BY o.order_id; ``` I'm using PostgreSQL version 13.2. When I execute this query, I see that the `user_count` is returning a much higher number than expected. I suspect that the scenario lies in how the joins are being processed, especially if there are multiple orders per user. To troubleshoot, I tried running the query without the `GROUP BY` clause, which returned a total count of users without the order-level granularity. This count seemed more aligned with my expectations. Additionally, I considered using a subquery to first get the distinct users per order like this: ```sql SELECT o.order_id, u.user_id FROM orders o JOIN users u ON o.user_id = u.user_id GROUP BY o.order_id, u.user_id; ``` Yet, I still end up with inflated counts when aggregating. I also tried adding filters to see if specific orders were causing the discrepancy, but the counts remained inconsistent. Is there a better way to structure this query to get an accurate count without inflating the results? Any insights on managing joins and grouping in PostgreSQL would be greatly appreciated! My development environment is Debian. How would you solve this?