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Comparing relational languages by their logical expressiveness is wellunderstood. Less well understood is how to compare relational languages bytheir ability to represent relational query patterns. Indeed, what are querypatterns other than "a certain way of writing a query"? And how can querypatterns be defined across procedural and declarative languages, irrespectiveof their syntax? To the best of our knowledge, we provide the first semanticdefinition of relational query patterns by using a variant ofstructure-preserving mappings between the relational tables of queries. Thisformalism allows us to analyze the relative pattern expressiveness ofrelational language fragments and create a hierarchy of languages with equallogical expressiveness yet different pattern expressiveness. Notably, for thenon-disjunctive language fragment, we show that relational calculus can expressa larger class of patterns than the basic operators of relational algebra.  Our language-independent definition of query patterns opens novel paths forassisting database users. For example, these patterns could be leveraged tocreate visual query representations that faithfully represent query patterns,speed up interpretation, and provide visual feedback during query editing. As aconcrete example, we propose Relational Diagrams, a complete and sounddiagrammatic representation of safe relational calculus that is provably (i)unambiguous, (ii) relationally complete, and (iii) able to represent all querypatterns for unions of non-disjunctive queries. Among all diagrammaticrepresentations for relational queries that we are aware of, ours is the onlyone with these three properties. Furthermore, our anonymously preregistereduser study shows that Relational Diagrams allow users to recognize patternsmeaningfully faster and more accurately than SQL.

On The Reasonable Effectiveness of Relational Diagrams: Explaining Relational Query Patterns and the Pattern Expressiveness of Relational Languages