I already got the UID and I wanna get email hashes from UID. I also can fetch FQL to get user's name and sex etc but when I SELECT email_hashes it returns empty array. Any help?
From Facebook's documentation:
An array containing a set of confirmed email hashes for the user. Emails are registered via the connect.registerUsers API call and are only confirmed when the user adds your application. The format of each email hash is the crc32 and md5 hashes of the email address combined with an underscore (_).
You don't get this for free just by getting an install - you only get them when you pass them in yourself via the connect.registerUsers API. If you want to e-mail a user, you'll need to get the email permission.