  We theoretically study the superconducting proximity effect in a quantum dot
coupled to two superconducting leads when the intradot interaction between
electrons is made attractive. Because of the superconducting proximity effect,
the electronic states for the embedded quantum dot are either spin-polarized
states with an odd occupation number or BCS-like states with an even occupation
number. We show that in the presence of an external magnetic field, the system
can exhibit quantum phase transitions of fermion parity associated with the
occupation number. In this work, we adopt a self-consistent theoretical method
to extend our considerations beyond the so-called superconducting atomic limit
in which the superconducting gap for the leads is assumed to be the largest
energy scale. The method enables us to numerically investigate the electronic
structure of the dot as results of the attractive interaction. For energy phase
diagrams in the regime away from the atomic limit, we find a reentrant behavior
where a BCS-like phase of the dot exists in an intermediate range of the
hybridization strength between the quantum dot and the leads. We also consider
Josephson current phase relations and identify a number of examples showing
$0-\pi$ phase transitions that may offer important switching effects.
