Dear Mustafa,
Yes, it's a single piece (group) -- but made up of several separate pieces (elements), cemented together...
You need more than one element to correct aberrations. The two classical routes are symetrical derivatives (many aberrations cancelling out automatically at infinity, even with a very simple 2-glass lens with glasses of the same refractive index, though normally each pair is itself a cemented doublet with the two components having different refractive indices) and triplet derivatives, always using (as far as I am aware) glasses of different refractive indices.
I think you need a book such as Cox's Optics to give you an overview of lens design.
Cheers,
R.