Sandy Springs, once a part of Atlanta, liberated themselves and are a self governing city now. Sandy Springs residents were paying huge taxes to the city of Atlanta and were not getting the services they paid for.
FWIW, I actually lived in Sandy Springs for more than a dozen years before relocating to FL after the kids all graduated from college. Sandy Springs was part of the unincorporated area of Fulton County. To the best of my knowledge, it was never part of Atlanta. It finally incorporated shortly before we bought our first home in the city, so right around early 2006 or 2005.