PostgreSQL: Why is my CTE returning rows with NULL values after filtering?
I'm working with a Common Table Expression (CTE) in PostgreSQL 14 to generate a list of active users based on their last login date. However, after filtering for users who logged in within the last 30 days, I'm still getting rows with NULL values for some of the users. My CTE looks like this: ```sql WITH active_users AS ( SELECT user_id, last_login FROM users WHERE last_login >= NOW() - INTERVAL '30 days' ) SELECT * FROM active_users; ``` When I run this query, I expect to only see users who have logged in during the last 30 days. However, I still receive rows with NULL in the `last_login` column. I’ve already checked the `users` table, and there are indeed rows with NULL values in the `last_login` field, but they shouldn’t be part of the result set. I also tried adding a filter directly in the main SELECT statement like this: ```sql WITH active_users AS ( SELECT user_id, last_login FROM users ) SELECT * FROM active_users WHERE last_login IS NOT NULL AND last_login >= NOW() - INTERVAL '30 days'; ``` This returns the same unexpected results. Additionally, I verified that my `last_login` column is indexed, but the performance is not an scenario here since the dataset is relatively small. Is there something about the CTE execution in PostgreSQL that I'm missing, or how can I ensure that my filtering works as expected? Any insights would be greatly appreciated! Am I approaching this the right way?