MySQL subquery returning different results when used with LIMIT in Laravel 8
I've been banging my head against this for hours. I keep running into I've searched everywhere and can't find a clear answer... I'm working on a personal project and I'm working with a perplexing scenario in my Laravel 8 application where a subquery returns inconsistent results when I apply a LIMIT clause. I have the following queries that are meant to retrieve the latest orders for each user from my `orders` table, which has a foreign key `user_id`. Here's the query I initially tried: ```sql SELECT * FROM orders o WHERE o.id IN ( SELECT id FROM orders WHERE user_id = o.user_id ORDER BY created_at DESC LIMIT 1 ); ``` However, the above query seems to be returning multiple latest orders for some users instead of just one. After some experimentation, I rewrote the query using a join: ```sql SELECT o1.* FROM orders o1 JOIN ( SELECT user_id, MAX(created_at) as latest_order FROM orders GROUP BY user_id ) o2 ON o1.user_id = o2.user_id AND o1.created_at = o2.latest_order; ``` This yielded the expected results, but I noticed it performed significantly slower on larger datasets. I've ensured that both `user_id` and `created_at` columns are indexed properly, but the performance is still not satisfactory. I'm using MySQL 8 and Laravel 8 with Eloquent ORM. Is there a more efficient way to fetch the latest order per user without running into performance optimization? Also, why does the initial subquery return multiple records when it should only return one per user? Any insights would be very helpful! My development environment is macOS. How would you solve this? How would you solve this? For reference, this is a production service. Has anyone dealt with something similar?