MySQL query using GROUP_CONCAT() in a subquery returns NULL instead of expected concatenated values
I'm having a hard time understanding I've been banging my head against this for hours. I've tried everything I can think of but I'm sure I'm missing something obvious here, but I'm working on a project and hit a roadblock. I'm working on a MySQL 8.0.26 project where I need to aggregate values from a related table, but when using `GROUP_CONCAT()` in a subquery, I'm getting NULL instead of the concatenated results. I have two tables: `students` and `courses`. The `students` table has columns `id`, `name`, and `course_id`, while the `courses` table has `id` and `course_name`. My aim is to get a list of each student along with a comma-separated list of the courses they are enrolled in. I wrote the following query: ```sql SELECT s.name, ( SELECT GROUP_CONCAT(c.course_name) FROM courses c WHERE c.id = s.course_id ) AS courses FROM students s; ``` However, this returns NULL for the `courses` column. Upon inspecting the `students` table, I see that `course_id` values do match the `id` values in the `courses` table. I even tried switching to an `INNER JOIN` instead, like this: ```sql SELECT s.name, GROUP_CONCAT(c.course_name) AS courses FROM students s INNER JOIN courses c ON c.id = s.course_id GROUP BY s.id; ``` This version works correctly and returns the expected results, but I would rather have it in the original subquery format. I suspect it might be related to how the subquery is being evaluated. Is there a reason why my first query is returning NULL, and is there a way to fix it while still using a subquery? Any insights would be greatly appreciated! For context: I'm using Sql on Linux. I'm working on a CLI tool that needs to handle this. I'm developing on Ubuntu 22.04 with Sql. Any feedback is welcome! For reference, this is a production microservice.