How to read author_name from current table (comments) if id_author is set to zero or from users table if id_author is bigger then 0(MySQL command)

Question

I have a comment form like this. -form with 1 field (comments) if user is login or -form with 3 fields (name, email, comment) if user is not login (like visitor)

After user is submitting the comment depending by status (login or not) write inside the comments table the data like this example: - if user is login write com_id_author=3 and keep empty com_author and com_email - if user is NOT login write com_id_author=0 , com_author="Andreas" , com_email="[email protected]"

comments

com_id | com_id_author | com_author (allow NULL) | com_email (allow NULL) | com_content

users

user_id | user_name | user_pass | user_email

My question is:

What is the MYSQL syntax to read: comment id, author id , author name, author email and content