MySQL: Unexpected results with LEFT JOIN and NULL handling in WHERE clause
I'm performance testing and Hey everyone, I'm running into an issue that's driving me crazy. I'm experiencing unexpected results when performing a LEFT JOIN in MySQL 8.0, particularly when I'm trying to filter results based on a condition that involves a NULL check. The query is supposed to return records from the `orders` table even if there is no corresponding record in the `customers` table. However, when I add a condition in the WHERE clause to filter based on `customers.name`, the results change unexpectedly. Here's the SQL query I'm using: ```sql SELECT o.order_id, c.name FROM orders o LEFT JOIN customers c ON o.customer_id = c.id WHERE c.name IS NULL; ``` I expected this query to return all orders that do not have a corresponding customer. However, it seems that the query is returning no results at all. After some investigation, I realized that adding the `WHERE` clause is filtering out the left-joined records, which is not what I intended. To troubleshoot, I tried running the query without the WHERE condition and got the expected results: ```sql SELECT o.order_id, c.name FROM orders o LEFT JOIN customers c ON o.customer_id = c.id; ``` This returns all orders, including those without customers, as intended. Additionally, I experimented with using `IS NOT NULL` and even tried moving the condition into the join clause: ```sql SELECT o.order_id, c.name FROM orders o LEFT JOIN customers c ON o.customer_id = c.id AND c.name IS NULL; ``` But that also didn't yield the expected results. I understand that the WHERE clause is evaluated after the JOIN, but I'm curious if there's a better way to achieve my goal without losing the left join functionality. Any insights on how to properly filter left-joined results while still retrieving the full set of orders would be greatly appreciated! For context: I'm using Sql on Windows 10. Thanks for taking the time to read this!