PostgreSQL: Handling duplicate rows in a join query with DISTINCT optimization guide as expected
I'm maintaining legacy code that I've been researching this but I'm wondering if anyone has experience with I'm relatively new to this, so bear with me... I'm working on a project and hit a roadblock. I'm currently working on a PostgreSQL 13.3 database and working with an scenario with a join query where I expect to eliminate duplicates using `DISTINCT`, but it doesn't seem to be working as anticipated. The query is joining two tables, `orders` and `customers`, based on a foreign key relationship. Here is the SQL query I've written: ```sql SELECT DISTINCT c.id, c.name FROM customers c JOIN orders o ON c.id = o.customer_id WHERE o.status = 'completed'; ``` However, I am still receiving duplicate customer IDs in the result set. I thought using `DISTINCT` would filter out any duplicates, but it seems to only be considering the unique combinations of `c.id` and `c.name`. To troubleshoot, I also tried grouping the results: ```sql SELECT c.id, c.name FROM customers c JOIN orders o ON c.id = o.customer_id WHERE o.status = 'completed' GROUP BY c.id, c.name; ``` This returned the same duplicates as before, and I believe my `GROUP BY` may not be resolving the scenario. I suspect it might be related to the fact that a single customer can have multiple completed orders. Is there a better approach to get a unique list of customers who have completed orders? Should I be considering any additional fields or modifying the join condition? Any help would be greatly appreciated! What am I doing wrong? My development environment is macOS. I'd really appreciate any guidance on this. This is my first time working with Sql 3.9. I'd be grateful for any help. Any ideas how to fix this? For reference, this is a production desktop app. Is there a simpler solution I'm overlooking? What's the best practice here?