In recent years we witnessed the increasing trend that more applications are developed by composing Web services. Services interact with one another in ways not necessarily foreseen during their development phase. In this setting, mismatches usually exist in the data flow of a Web service composition, and mediators are typically required to reconcile potential conflicts. In this paper we propose a service-based approach for automatically inserting appropriate mediation services in Data-as-a-Service (DaaS) compositions to resolve structural heterogeneities in their data flow. The mediation is ensured through a special kind of Web services called Data Mapping Web services. We also define a distributed solution to publish/discover these services. Provided experimental evaluations in this paper show that our approach is efficient in realistic situations.