SQL Server: Unexpected NULL results when using COALESCE with LEFT JOIN on filtered subquery
I keep running into Can someone help me understand After trying multiple solutions online, I still can't figure this out... I'm working with an scenario with a `LEFT JOIN` where I'm using `COALESCE` to handle potential NULLs. The goal is to join a filtered subquery against a main table and return a default value if there are no matches. However, I'm getting unexpected NULL results instead of the default value I expect. Here's the relevant SQL query: ```sql SELECT a.id, COALESCE(b.value, 'default') AS value FROM main_table a LEFT JOIN ( SELECT * FROM related_table WHERE condition = 'value' ) b ON a.id = b.main_id; ``` In this example, `main_table` contains records with IDs, and `related_table` has a reference to those IDs. When there are no matching records in `related_table` that meet the condition, I expect `COALESCE` to return 'default'. Instead, I occasionally get NULL for some `a.id` values that should match. I've tried checking for other filters that might be affecting the join, but nothing seems to point to an scenario. Additionally, I verified that the `condition` in the subquery does not exclude any valid relationships. I also ran a separate query to confirm expected values exist in `related_table`: ```sql SELECT * FROM related_table WHERE condition = 'value'; ``` This returns expected rows. Is there anything I'm overlooking in the way the LEFT JOIN operates with the filtered subquery, or do you have any insights on how to troubleshoot this further? I'm using SQL Server 2019, and I've confirmed there are no indexing issues impacting performance. This is part of a larger API I'm building. What am I doing wrong? I'm coming from a different tech stack and learning Sql. Thanks for taking the time to read this! I've been using Sql for about a year now. Is there a simpler solution I'm overlooking?