PostgreSQL 14: Getting Unexpected NULLs When Using LEFT JOIN with COALESCE
This might be a silly question, but I'm experiencing an issue with a LEFT JOIN in PostgreSQL 14 where the resulting dataset contains unexpected NULL values. I have two tables, `orders` and `customers`. The goal is to retrieve all orders along with the corresponding customer names, using COALESCE to provide a default value in case the customer is not found. Here's the SQL query I've written: ```sql SELECT o.order_id, COALESCE(c.customer_name, 'Unknown') AS customer_name FROM orders o LEFT JOIN customers c ON o.customer_id = c.customer_id; ``` However, I'm getting `NULL` for `customer_name` in some rows where I expect to see 'Unknown'. I've double-checked that the `customer_id` in the `orders` table is indeed matching with the `customers` table, but it seems like some rows are being returned without a match, leading to the NULL output. I also tried using a WHERE clause to filter out any orders with NULL customer_ids, but that didn't help. Here's the modified query: ```sql SELECT o.order_id, COALESCE(c.customer_name, 'Unknown') AS customer_name FROM orders o LEFT JOIN customers c ON o.customer_id = c.customer_id WHERE o.customer_id IS NOT NULL; ``` This also resulted in unexpected NULLs in `customer_name`. I ran the following checks: 1. Verified that all `customer_id` values in `orders` actually exist in `customers`. 2. Checked for any trailing spaces or formatting issues in the `customer_id` fields. 3. Ensured that the `customer_id` types in both tables are the same. Is there something I'm overlooking in my query or data that would cause this behavior? Any guidance would be greatly appreciated! What's the best practice here?