Here is my code:

```
SELECT COALESCE(CONCAT(u.user_fname, ' ', u.user_lname), 'unknown') name
FROM users u
WHERE id = 10;

```

The result will be `unknown` when either `user_fname` or `user_lname` is null. That's not what I want, I want to select `unknown` only when both `user_fname` and `user_lname` are null. 

Otherwise, I want to get the value of not-null column. How can I do that?