Assalaamualaykum,
Is it permissible to have an egg as food?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.

You did not clarify to us which kind of eggs you meant. In any case, in general the eggs of an animal the meat of which is lawful to eat, is permissible to eat, and the eggs of an animal the meat of which is not lawful to eat, is forbidden to eat. Also, the scholars forbade eating the eggs of a pure animal, which turned to blood (became rotten), and they differed in opinion about the eggs of an animal that is lawful to eat but which feeds on impurity and its flesh became impure. 

Allaah Knows best.