Fill in the login field with your username OR email

I built a register and login system on my website. Now I asked myself a question that I cannot answer. I have three inputs on my registration page:

1. Username
2. Email
3. Password

I would now like to have two inputs on my login page instead. The first should be for password and the second for email OR username. I now wonder how it is possible to do this with the MySQL database?

I am still very new to PHP. I hope someone can answer, I haven't found anything on Stack Overflow.