  We show that, by using unit-cell transformation between 1 Fe per unit cell to
2 Fe per unit cell, one can qualitatively understand the pairing symmetry of
several families of iron-based superconductors. In iron-pnictides and
iron-chalcogenides, the nodeless s+- pairing and the resulting magnetic
resonance mode transform nicely between the two unit cells, while retaining all
physical properties unchanged. However, when the electron-pocket disappears
from the Fermi surface with complete doping in KFe2As2, we find that the
unit-cell invariant requirement prohibits the occurrence of s+- pairing
symmetry (caused by inter-hole-pocket nesting). However, the intra-pocket
nesting is compatible here, which leads to a nodal d-wave pairing. The
corresponding Fermi surface topology and the pairing symmetry are similar to
Ce-based heavy-fermion superconductors. Furthermore, when the Fermi surface
hosts only electron-pockets in KyFe2-xSe2, the inter-electron-pocket nesting
induces a nodeless and isotropic d-wave pairing. This situation is analogous to
the electron-doped cuprates, where the strong antiferromagnetic order creates
similar disconnected electron-pocket Fermi surface, and hence nodeless d-wave
pairing appears. The unit-cell transformation in KyFe2-xSe2 exhibits that the
d-wave pairing breaks the translational symmetry of the 2 Fe unit cell, and
thus cannot be realized unless a vacancy ordering forms to compensate for it.
These results are consistent with the coexistence picture of a competing order
and nodeless d-wave superconductivity in both cuprates and KyFe1.6Se2.
