PostgreSQL: Getting NULL instead of aggregated values when using LEFT JOIN with GROUP BY
I've encountered a strange issue with I'm working on a project and hit a roadblock... I've been struggling with this for a few days now and could really use some help. I'm working with an scenario with a LEFT JOIN in PostgreSQL where I'm expecting to get aggregated values but instead I'm getting NULL for some rows. I have two tables: `orders` and `customers`. The `orders` table has an `order_id`, `customer_id`, and `amount`, while the `customers` table has `customer_id` and `customer_name`. My goal is to retrieve the total order amount for each customer even if they haven't placed any orders. Here's the query I've written: ```sql SELECT c.customer_name, SUM(o.amount) AS total_amount FROM customers c LEFT JOIN orders o ON c.customer_id = o.customer_id GROUP BY c.customer_name; ``` This returns the expected results for customers who have placed orders, but for those who haven't, I am getting NULL for `total_amount` instead of 0. The output looks like this: | customer_name | total_amount | |---------------|--------------| | John Doe | 150.00 | | Jane Smith | NULL | | Alice Johnson | 200.00 | I tried using `COALESCE` to handle the NULL values like this: ```sql SELECT c.customer_name, COALESCE(SUM(o.amount), 0) AS total_amount FROM customers c LEFT JOIN orders o ON c.customer_id = o.customer_id GROUP BY c.customer_name; ``` However, I'm still getting NULL instead of 0 for `total_amount` for customers without orders. I also verified that all customers exist in the `customers` table. I'm using PostgreSQL version 13.5. What am I missing here? Any insights would be greatly appreciated! For context: I'm using Sql on Windows. Is there a better approach? Am I missing something obvious? For context: I'm using Sql on Ubuntu. What's the best practice here? What's the best practice here? Thanks for taking the time to read this!