“…Keys and uniqueness constraints have also been studied in various different data models, including Web models such as XML [23], JSON [37] and RDF [30], models where data is nested using records, lists, sets, or union operators [38,41], description logics [44], temporal models [26,50], objectrelational [28] and object-oriented data models [7], as well as models for uncertain data such as probabilistic databases [9] or possibilistic databases [3].…”