Correct HTTP status code for existent resource, but non-existent entity?

Question

Say the client is requesting the following URL:

/user-details?user=123

If /user-details was a non-existing resource, the corre

Answer 1

The 404 is fine because the user-details resource is a conceptual mapping to the user entity in this case to a partial user resource information.

The GET method for user-details is therefore not responsible for differentiating from the two cases: a) The user doesn't exist, b) The user details don't exist.

I would however rewrite the endpoint to something like this:

/user/123/details

Which in my opinion is more expressive.