UPDATE default_comments c
INNER JOIN default_profiles p ON c.user_id = p.user_id 
INNER JOIN default_users u ON c.user_id = u.id 
SET c.name = p.display_name,
c.email = u.email,
c.website = p.website
WHERE c.name = '';

Update an empty name column in default_comments

UPDATE table using JOINS

mysql

MySQL