Show query results for keys not found [MySQL]

Question

IF I am comparing 2 tables users [user_id, username] and user_comments [user_id, comment] where user_id are the

Answer 1

I think you want a left join:

select coalesce(cast(uc.user_id as varchar(255)), 'unknown') as user_id,
       uc.comment
from user_comments uc left join
     users u
     on u.user_id = uc.user_id;

You do need to be careful about the type of user_id. Assuming it is not a string, you need to cast it to be compatible with 'unknown'. Otherwise, NULL is a fine non-matching value.