The goal of the Web Service Composition (WSC) problem is to find an optimal composition of Web Services to satisfy a given request using their syntactic and/or semantic features, when no single service satisfies it. However in many cases such composition is impossible due for example to a bad formulation from the user. In this paper, we consider WSC problem in the Software as a Service layer (SaaS) from semantic aspects, exploiting the super-type and subtype relationship among parameters. In this paper we investigate the problem of SaaS Web Service Composition. We formulate the problem of optimal composition as the problem of finding the minimal unsatisfiable set of minimum size widely studied in the context of Artificial Intelligence that encoding the composition problem. Furthermore, we proved that our framework can be extended to take into account the quality of services.