OBDA for the Web: Creating Virtual RDF Graphs On Top of Web Data Sources

Efficient Handling of SPARQL OPTIONAL for OBDA (Extended Version)

OPTIONAL is a key feature in SPARQL for dealing with missing information. While this operator is used extensively, it is also known for its complexity, which can make efficient evaluation of queries with OPTIONAL challenging. We tackle this problem in the Ontology-Based Data Access (OBDA) setting, where the data is stored in a SQL relational database and exposed as a virtual RDF graph by means of an R2RML mapping. We start with a succinct translation of a SPARQL fragment into SQL. It fully respects bag semantics and three-valued logic and relies on the extensive use of the LEFT JOIN operator a…

Enhancing Virtual Ontology Based Access over Tabular Data with Morph-CSV

Ontology-Based Data Access (OBDA) has traditionally focused on providing a unified view of heterogeneous datasets, either by materializing integrated data into RDF or by performing on-the fly querying via SPARQL query translation. In the specific case of tabular datasets represented as several CSV or Excel files, query translation approaches have been applied by considering each source as a single table that can be loaded into a relational database management system (RDBMS). Nevertheless, constraints over these tables are not represented; thus, neither consistency among attributes nor indexes …

Creating RESTful APIs over SPARQL endpoints using RAMOSE

Semantic Web technologies are widely used for storing RDF data and making them available on the Web through SPARQL endpoints, queryable using the SPARQL query language. While the use of SPARQL endpoints is strongly supported by Semantic Web experts, it hinders broader use of RDF data by common Web users, engineers and developers unfamiliar with Semantic Web technologies, who normally rely on Web RESTful APIs for querying Web-available data and creating applications over them. To solve this problem, we have developed RAMOSE, a generic tool developed in Python to create REST APIs over SPARQL end…

Foundations of an Alternative Approach to Reification in RDF

This document defines extensions of the RDF data model and of the SPARQL query language that capture an alternative approach to represent statement-level metadata. While this alternative approach is backwards compatible with RDF reification as defined by the RDF standard, the approach aims to address usability and data management shortcomings of RDF reification. One of the great advantages of the proposed approach is that it clarifies a means to (i) understand sparse matrices, the property graph model, hypergraphs, and other data structures with an emphasis on link attributes, (ii) map such da…

Creating Knowledge Graphs Subsets using Shape Expressions

The initial adoption of knowledge graphs by Google and later by big companies has increased their adoption and popularity. In this paper we present a formal model for three different types of knowledge graphs which we call RDF-based graphs, property graphs and wikibase graphs. In order to increase the quality of Knowledge Graphs, several approaches have appeared to describe and validate their contents. Shape Expressions (ShEx) has been proposed as concise language for RDF validation. We give a brief introduction to ShEx and present two extensions that can also be used to describe and validate …

SP2Bench：一个SPARQL性能基准

SP2Bench: A SPARQL Performance Benchmark

最近，用于RDF的SPARQL查询语言已经达到了W3C推荐标准。为了响应这个新兴的标准，数据库社区目前正在探索RDF数据的有效存储技术和SPARQL查询的评估策略。要对这些方法进行有意义的分析和比较，就必须有一个全面和普遍的基准平台。为此，我们开发了SP^2Bench，这是一个公开可用的、特定于语言的SPARQL性能基准。SP^2Bench是DBLP场景中的解决方案，它包括一个用于创建任意大的类DBLP文档的数据生成器和一组精心设计的基准查询。生成的文档反映了原始DBLP数据集中遇到的关键特征和社交世界分布，而查询则在此数据之上实现了有意义的请求，涵盖了各种SPARQL操作符星座和RDF访问模式。作为概念验证，我们将SP^2Bench应用于现有引擎，并讨论其优势和劣势，这些优势和劣势直接来自基准测试结果。