We have theoretically studied the electronic and optical properties of a quantum well (QW) in which the well region is constructed from a binary alloy semiconductor A(1-x)B(lambda) in the coherent potential approximation (CPA). A tight binding model is used for a single particle (electron, hole, Frenkel exciton) in the well composed by a rectangular array of N-x x N-y x N-z sites. The effect of the diagonal randomness is reduced to the coherent potential Sigma(E), which is assumed to be the same for all sites, and is selfconsistently determined with the average Green's function. For a slab (infinity, infinity, N-z) and wire (infinity, N-y, N-z), the density of states Q(E) is composed of N-z (or N-y x N-z) subbands, each shows the two (one)-dimensional van-Hove singularity. When x (or 1 - x) is small, a B (A) impurity-band always appears at the lower (higher) energy side of the lowest (highest) host-band. As the welt width becomes narrower and/or the dimensionality decreases, the boundary for Delta/t decreases which separates the amalgamation type and the persistence type.