With the growing amount of data being continuously produced, it is crucial to come up with solutions to expose data from ever more heterogeneous databases (e.g. NoSQL systems) as linked data. In this paper we present xR2RML, a language designed to describe the mapping of various types of databases to RDF. xR2RML flexibly adapts to heterogeneous query languages and data models while remaining free from any specific language or syntax. It extends R2RML, the W3C recommendation for the mapping of relational databases to RDF, and relies on RML for the handling of various data representation formats. We analyse data models of several modern databases as well as the format in which query results are returned, and we show that xR2RML can translate any data element within such results into RDF, relying on existing languages such as XPath and JSONPath if needed. We illustrate some features of xR2RML such as the generation of RDF collections and containers, and the ability to deal with mixed content.