Can I install Windows 7 on an external drive and boot

[WatchTower]

I was wondering if I can boot from an external drive on my 2011 Macbook Pro. I'd prefer to do that than share my Mac OS hard drive with Windows. I've been searching finding nothing really up to date.

 

[Skyboat]

Well it's definitely possible as I've been able to boot up my old HDD when I put it into a USB caddy after an upgrade, but I don't know the exact specifics.

What happens if you keep a USB drive plugged in and boot up the Windows installation disc?