come by something—
1. obtain smth. (that is hard to get):
- Jobs were not so easy to come by when I was a boy.
2. happen to get smth. by accident:
- How did you come by that scratch on your cheek?
go by something—
1. act according to smth.:
- If we go by instructions, we must leave now.
2. base one’s judgment on smth.:
- You make a mistake if you go by appearances.