# To plot 3D graph with a region boundary as the constraint [duplicate]

How to get the output graph which consider only x^2+y^2=4 (the boundary of the region only)? I found this on the Help.

Plot3D[{x^2 + y^2, -x^2 - y^2}, {x, -2, 2}, {y, -2, 2},
RegionFunction -> Function[{x, y, z}, x^2 + y^2 <= 4],
BoxRatios -> Automatic]


When I change <= to ==, no output comes. Or do I need to use different command?

Plot3D[{x^2 + y^2, -x^2 - y^2},