Hii praveen...
Value of m for dz2 is 0 because  of following
m stands for magnetic qunatum number.
Magnetic quantum number explains orientation of orbit in the space.
Values of m for other subshells -
For d(xy), m = -2
For d(yz), m = -1
For d(z2), m = 0
For d(zx), m = +1
For d(x2-y2), m = +2
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