  The development of practical query languages for graph databases runs well
ahead of the underlying theory. The ISO committee in charge of database query
languages is currently developing a new standard called Graph Query Language
(GQL) as well as an extension of the SQL Standard for querying property graphs
represented by a relational schema, called SQL/PGQ. The main component of both
is the pattern matching facility, which is shared by the two standards. In many
aspects, it goes well beyond RPQs, CRPQs, and similar queries on which the
research community has focused for years. Our main contribution is to distill
the lengthy standard specification into a simple Graph Pattern Calculus (GPC)
that reflects all the key pattern matching features of GQL and SQL/PGQ, and at
the same time lends itself to rigorous theoretical investigation. We describe
the syntax and semantics of GPC, along with the typing rules that ensure its
expressions are well-defined, and state some basic properties of the language.
With this paper we provide the community a tool to embark on a study of query
languages that will soon be widely adopted by industry.
