MySQL 5.7: Strange behavior when using GROUP BY with aggregate functions and NULLs in a subquery
I'm facing an issue with MySQL 5.7 where using `GROUP BY` alongside aggregate functions and `NULL` values in a subquery seems to lead to unexpected results. I have a table called `sales` with the following structure: ```sql CREATE TABLE sales ( id INT PRIMARY KEY, product_id INT, quantity INT, sale_date DATE ); ``` I want to get the total quantity sold for each product, including those products that have never been sold (i.e., products with `NULL` quantity). Here's the query I wrote: ```sql SELECT p.id AS product_id, SUM(s.quantity) AS total_quantity FROM products p LEFT JOIN (SELECT product_id, SUM(quantity) AS quantity FROM sales GROUP BY product_id) s ON p.id = s.product_id GROUP BY p.id; ``` However, I'm getting `NULL` for `total_quantity` for products that should show `0`, which is not what I expected. I've tried using `COALESCE(s.quantity, 0)` but it doesn't seem to resolve the issue. The output looks like this: ``` +------------+---------------+ | product_id | total_quantity | +------------+---------------+ | 1 | NULL | | 2 | 10 | | 3 | NULL | +------------+---------------+ ``` I also tried changing the subquery to include `COALESCE` directly there, but the behavior remains the same: ```sql SELECT p.id AS product_id, COALESCE(SUM(s.quantity), 0) AS total_quantity FROM products p LEFT JOIN (SELECT product_id, COALESCE(SUM(quantity), 0) AS quantity FROM sales GROUP BY product_id) s ON p.id = s.product_id GROUP BY p.id; ``` The output still shows `NULL` values for products that haven't been sold. I need to ensure that all products appear in the result set, with `0` instead of `NULL` for those that haven't sold anything. Has anyone encountered this issue or can suggest a different approach? My development environment is macOS.