  Query formulation is increasingly performed by systems that need to guess a
user's intent (e.g. via spoken word interfaces). But how can a user know that
the computational agent is returning answers to the "right" query? More
generally, given that relational queries can become pretty complicated, how can
we help users understand existing relational queries, whether human-generated
or automatically generated? Now seems the right moment to revisit a topic that
predates the birth of the relational model: developing visual metaphors that
help users understand relational queries.
  This lecture-style tutorial surveys the key visual metaphors developed for
visual representations of relational expressions. We will survey the history
and state-of-the art of relationally-complete diagrammatic representations of
relational queries, discuss the key visual metaphors developed in over a
century of investigating diagrammatic languages, and organize the landscape by
mapping their used visual alphabets to the syntax and semantics of Relational
Algebra (RA) and Relational Calculus (RC).
