List of emails of users watching a specific category

Community Building Data & reporting
2

You need to join the user_emails table based on the user_id from the category_users table. Give something like this a try:

SELECT
cu.*,
ue.email
FROM category_users cu
JOIN user_emails ue
ON ue.user_id = cu.user_id
WHERE category_id = '10'
AND ue.primary = true