When querying an Resource Description Framework (RDF) graph, a prominent feature is the possibility of extending the answer to a query with optional information. However, the definition of this feature in SPARQL—the standard RDF query language—has raised some important issues. Most notably, the use of this feature increases the complexity of the evaluation problem, and its closed-world semantics is in conflict with the underlying open-world semantics of RDF. Many approaches for fixing such problems have been proposed, the most prominent being the introduction of the semantic notion of weakly monotone SPARQL query. Weakly monotone SPARQL queries have shaped the class of queries that conform to the open-world semantics of RDF. Unfortunately, finding an effective way of restricting SPARQL to the fragment of weakly monotone queries has proven to be an elusive problem. In practice, the most widely adopted fragment for writing SPARQL queries is based on the syntactic notion of well designedness. This notion has proven to be a good approach for writing SPARQL queries, but its expressive power has yet to be fully understood. The starting point of this article is to understand the relation between well-designed queries and the semantic notion of weak monotonicity. It is known that every well-designed SPARQL query is weakly monotone; as our first contribution we prove that the converse does not hold, even if an extension of this notion based on the use of disjunction is considered. Given this negative result, we embark on the task of defining syntactic fragments that are weakly monotone and have higher expressive power than the fragment of well-designed queries. To this end, we move to a more general scenario where infinite RDF graphs are also allowed, so interpolation techniques studied for first-order logic can be applied. With the use of these techniques, we are able to define a new operator for SPARQL that gives rise to a query language with the desired properties (over finite and infinite RDF graphs). It should be noticed that every query in this fragment is weakly monotone if we restrict the semantics to finite RDF graphs. Moreover, we use this result to provide a simple characterization of the class of monotone CONSTRUCT queries, that is, the class of SPARQL queries that produce RDF graphs as output. Finally, we pinpoint the complexity of the evaluation problem for the query languages identified in the article.

中文翻译：

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为 RDF 设计查询语言

在查询资源描述框架 (RDF) 图时，一个突出的特点是可以将答案扩展到带有可选信息的查询。然而，在标准 RDF 查询语言 SPARQL 中对这一特性的定义引发了一些重要问题。最值得注意的是，该特性的使用增加了评估问题的复杂性，并且其封闭世界语义与 RDF 底层的开放世界语义相冲突。已经提出了许多解决此类问题的方法，最突出的是引入了弱单调 SPARQL 查询的语义概念。弱单调 SPARQL 查询塑造了符合 RDF 开放世界语义的查询类别。很遗憾，找到一种将 SPARQL 限制为弱单调查询片段的有效方法已被证明是一个难以捉摸的问题。在实践中，用于编写 SPARQL 查询的最广泛采用的片段是基于良好设计的句法概念。这个概念已被证明是编写 SPARQL 查询的好方法，但它的表达能力尚未被完全理解。本文的出发点是理解精心设计的查询与弱单调性语义概念之间的关系。众所周知，每个精心设计的 SPARQL 查询都是弱单调的；作为我们的第一个贡献，我们证明了逆不成立，即使考虑了基于析取使用的这个概念的扩展。鉴于这个负面结果，我们着手定义弱单调且比精心设计的查询片段具有更高表达能力的句法片段。为此，我们转向更一般的场景，其中也允许无限 RDF 图，因此可以应用为一阶逻辑研究的插值技术。通过使用这些技术，我们能够为 SPARQL 定义一个新的运算符，该运算符产生具有所需属性的查询语言（在有限和无限 RDF 图上）。应该注意的是，如果我们将语义限制为有限 RDF 图，则该片段中的每个查询都是弱单调的。此外，我们使用这个结果来提供单调 CONSTRUCT 查询类的简单表征，即生成 RDF 图作为输出的 SPARQL 查询类。最后，