MySQL 8.0 - Strange implementing INNER JOIN and NULL Values in Conditional Aggregations
I tried several approaches but none seem to work... Hey everyone, I'm running into an issue that's driving me crazy. I've been working on this all day and I'm working with a puzzling scenario when using conditional aggregations along with INNER JOIN in MySQL 8.0. I have two tables, `orders` and `customers`. The `orders` table includes an `amount` field and a `customer_id` field, while the `customers` table has a `customer_id` and a `status` field. My goal is to calculate the total amount of orders for each active customer, but I'm getting unexpected results, especially when the `orders` table has NULL values for the `amount`. Here's the SQL query I've written: ```sql SELECT c.customer_id, SUM(CASE WHEN o.amount IS NOT NULL THEN o.amount ELSE 0 END) AS total_amount FROM customers c INNER JOIN orders o ON c.customer_id = o.customer_id WHERE c.status = 'active' GROUP BY c.customer_id; ``` The question arises when I have active customers who haven't placed any orders; they are not included in the result set at all. I thought that by using an INNER JOIN, it would automatically exclude customers without orders, but I was expecting to see them with a total amount of `0`. When I replace the INNER JOIN with a LEFT JOIN, I do see those customers, but the aggregation of `NULL` amounts gives me unexpected results, showing `NULL` instead of `0`. I tried tweaking the query and used `COALESCE` to handle the `NULL` values: ```sql SELECT c.customer_id, COALESCE(SUM(o.amount), 0) AS total_amount FROM customers c LEFT JOIN orders o ON c.customer_id = o.customer_id WHERE c.status = 'active' GROUP BY c.customer_id; ``` This approach works better, and now I see those customers, but the total amount is still incorrect. I need to ensure that orders with NULL values don't affect the sum. Any insights on how to properly aggregate this data while ensuring that customers without orders show up with a total of `0`? Also, are there specific best practices I should follow when dealing with such aggregations in MySQL? This is part of a larger web app I'm building. Any help would be greatly appreciated! I'm using Sql 3.10 in this project. Any advice would be much appreciated. I'm using Sql 3.9 in this project. How would you solve this?