Relational databases scattered over the web are generally opaque to regular web crawling tools. To address this concern, many RDB-to-RDF approaches have been proposed over the last years. In this paper, the authors propose a detailed review of RDB-to-RDF methods and tools, considering end-to-end projects that delivered operational tools. The different approaches are classified along four major axes: motivation of the approach; mapping description language and expressiveness; mapping implementation; and data retrieval method. Then, they analyze commonalities and differences between existing approaches, and they underline common patterns in the orchestration of solutions applicable to each step of the translation process.