MySQL 8.0 - Unexpected Results When Using COALESCE with Subqueries in SELECT Statements
I've spent hours debugging this and I'm currently working on a MySQL 8.0 database where I'm trying to fetch user data along with their most recent order. For this, I'm using a subquery in combination with the `COALESCE` function. However, I'm encountering unexpected results where `NULL` values are not being handled correctly, leading to incorrect data being returned. Here’s the query I’m using: ```sql SELECT u.id, u.name, COALESCE((SELECT o.order_date FROM orders o WHERE o.user_id = u.id ORDER BY o.order_date DESC LIMIT 1), 'No Orders') AS last_order_date FROM users u; ``` The intention here is to get the latest order date for each user, and if a user has no orders, to return 'No Orders'. However, for a user that doesn't have any related orders, I sometimes get `NULL` instead of 'No Orders'. I also tried changing the default string in `COALESCE`, like so: ```sql COALESCE((SELECT o.order_date FROM orders o WHERE o.user_id = u.id ORDER BY o.order_date DESC LIMIT 1), NULL) ``` When I run the query, the output shows `NULL` for some users without orders, which is confusing since I thought `COALESCE` should take care of that. I’ve also checked to ensure that the `user_id` in the `orders` table matches the `id` in the `users` table correctly. I tried running the subquery standalone to verify it returns the expected results, and it does return `NULL` for users without orders, but it seems my outer query isn’t behaving as expected. Has anyone encountered similar behavior or have suggestions on how to resolve this? Any insights into why this might be happening or alternative approaches would be appreciated! I'm coming from a different tech stack and learning Sql. Any ideas what could be causing this?