ANSWER:

1. is it necessary to configure main domain before configuring subdomain?

No.

A useful point is that the first defined site is the one that is served if the client request is for a site domain name or address not in the Apache configuration explicitly.

1. can we configure only subdomain on server?

Yes.

You’ll need to arrange in DNS that the subdomain’s address records point to the appropriate web-server.

by RedGrittyBrick from http://superuser.com/questions/417631