MySQL 8.0: Unexpected Query Results with LEFT JOIN and COALESCE on NULL Values
I'm working through a tutorial and I tried several approaches but none seem to work. I'm maintaining legacy code that I'm trying to configure I'm currently working with MySQL 8.0 and encountered an scenario while trying to retrieve data using a LEFT JOIN with the COALESCE function... In my case, I'm attempting to join two tables: `users` and `orders`. My goal is to fetch all users along with their latest order if one exists, and display a default message if they have no orders. Here’s the query I’m using: ```sql SELECT u.id, u.name, COALESCE(o.order_date, 'No Orders') AS latest_order FROM users u LEFT JOIN ( SELECT user_id, MAX(order_date) AS order_date FROM orders GROUP BY user_id ) o ON u.id = o.user_id; ``` While the query executes without any errors, I noticed that the results for some users show 'No Orders' when they actually have orders in the `orders` table. I checked the data and confirmed that the users do have entries in the `orders` table. Here’s a snippet of the data: ```sql -- users table +----+----------+ | id | name | +----+----------+ | 1 | Alice | | 2 | Bob | | 3 | Charlie | +----+----------+ -- orders table +----+----------+------------+ | id | user_id | order_date | +----+----------+------------+ | 1 | 1 | 2023-10-01 | | 2 | 1 | 2023-10-05 | | 3 | 2 | 2023-10-03 | +----+----------+------------+ ``` The scenario seems to arise specifically with users who have multiple orders. The LEFT JOIN should return the correct order date for Alice, but instead, it shows 'No Orders'. I’ve tried running the inner query separately, and it returns the correct maximum order date, so I'm puzzled as to why the outer query doesn't reflect that. I've also checked for any potential NULL values in the `user_id` field of the `orders` table, and everything looks correct. Is there something I might be overlooking in the JOIN condition or the COALESCE usage? Any insights or suggestions would be greatly appreciated! I'm working on a web app that needs to handle this. How would you solve this? Could this be a known issue? Any ideas what could be causing this? This is my first time working with Sql latest. Is there a simpler solution I'm overlooking? I'm working with Sql in a Docker container on CentOS. I appreciate any insights!