  Relational databases have always had a means for creating a pseudo-table,
called a view, defined by a query. Views are like tables in most ways, except
that they are read-only and cannot be updated. The problem of how to update
views has attracted a lot of attention in the 1980s but is unsolved.
  The best approach from that time was by Bancilhon and Spyratos. I use one of
their overlooked theorems and find a number of simple solutions for common
relational operators.
