MySQL: Unexpected results when using GROUP_CONCAT with ORDER BY inside a subquery
I'm converting an old project and I'm encountering an issue with MySQL where I'm trying to use `GROUP_CONCAT` in a subquery to concatenate values from a related table, but the results are not coming out as expected. I'm using MySQL 8.0.23. The idea is to gather all the tags associated with a post and order them alphabetically within each group. Here is the query I'm using: ```sql SELECT p.id, p.title, (SELECT GROUP_CONCAT(t.name ORDER BY t.name) FROM tags t JOIN post_tags pt ON t.id = pt.tag_id WHERE pt.post_id = p.id) AS tag_list FROM posts p; ``` However, I am getting results where the `tag_list` column is returning duplicated tags or not returning them in the expected order. In some cases, it even returns NULL for some posts that definitely have tags. To troubleshoot, I’ve checked the data in both the `tags` and `post_tags` tables and everything seems fine. I've also tried running the inner query separately and confirmed that it works as intended when isolated. Additionally, I ensured that the relationships between posts and tags are set up correctly with foreign keys. As a last resort, I tried using the following simpler version of the query: ```sql SELECT p.id, p.title, GROUP_CONCAT(t.name ORDER BY t.name) AS tag_list FROM posts p LEFT JOIN post_tags pt ON p.id = pt.post_id LEFT JOIN tags t ON pt.tag_id = t.id GROUP BY p.id; ``` This version works better, but I’m wondering why the subquery approach doesn't yield the expected results and if there's a best practice for using `GROUP_CONCAT` in this way. Any insights would be appreciated! This issue appeared after updating to Sql stable. What's the best practice here?