Linked Open Data (LOD) is a successfully initiative Semantic Web, which uses URIs and RDF techniques to connect related pieces of data on the Semantic Web. A rapidly growing number of LODs leads to new challenges for query processing. SPARQL-endpoint has been proposed to access LODs through SPARQL language, however, it requires a holistic understanding of the data when making a SPARQL query and supports a limited number of related LODs. To access distributed LODs, the LOD-federation is proposed. The LOD-federation is an abstract layer consisting a federated ontology and mechanisms for user query processing across LODs via the corresponding endpoints. In this study, a technical survey of recent works supporting for the LOD-federation building, is presented. An effective method of LOD ontology integration is suggested in order to build the federated ontology. Query decomposition algorithm is depicted to translate a query to the LOD-federation into subsequent LODs. Result integration method using co-reference entity and join operations, is also presented.
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