On the semantics of Cypher's implicit group-by

Cypher is a popular declarative query language for property graphs. Despite having been adopted by several graph database vendors, it lacks a comprehensive semantics other than the reference implementation. This paper stems from Cypher.PL, a project aimed at creating an executable (and readable) semantics of Cypher in Prolog, and focuses on Cypher's implicit group-by feature. Rather than being explicitly specified in the query, in Cypher the grouping key is derived from the return expressions. We show how this becomes problematic when a single return expression mixes unaggregated property references and aggregating functions, and discuss ways of giving this construct a proper semantics without defying common sense.