Service Oriented Computing: Opportunities and Challenges -- Data Semantics Revisited -- Dynamic Agent Composition from Semantic Web Services -- Ontology-Extended Component-Based Workflows : A Framework for Constructing Complex Workflows from Semantically Heterogeneous Software Components -- Data Procurement for Enabling Scientific Workflows: On Exploring Inter-ant Parasitism -- XSDL: Making XML Semantics Explicit -- Refining Semantic Mappings from Relational Tables to Ontologies -- Triadic Relations: An Algebra for the Semantic Web -- Semantically Unlocking Database Content Through Ontology-Based Mediation -- Representation and Reasoning About Changing Semantics in Heterogeneous Data Sources -- Context Mediation in the Semantic Web: Handling OWL Ontology and Data Disparity Through Context Interchange -- HCOME: A Tool-Supported Methodology for Engineering Living Ontologies -- Query Answering by Rewriting in GLAV Data Integration Systems Under Constraints -- Utilizing Resource Importance for Ranking Semantic Web Query Results -- Querying Faceted Databases -- Constructing and Querying Peer-to-Peer Warehouses of XML Resources.This book constitutes the thoroughly refereed postproceedings of the Second International Workshop on Semantic Web and Databases, SWDB 2004, held in Toronto, Canada in August 2004 as a satellite workshop of VLDB 2004. The 14 revised full papers presented together with 2 papers by the invited keynote speakers were carefully selected during two rounds of reviewing and improvement from 47 submissions. Among the topics addressed are data semantics, semantic Web services, service-oriented computing, workflow composition, XML semantics, relational tables, ontologies, semantic Web algebra, heterogeneous data sources, context mediation, OWL, ontology engineering, data integration, semantic Web queries, database queries, and peer-to-peer warehouses.