The semijoin algebra and the guarded fragment
  The semijoin algebra is the variant of the relational algebra obtained by replacing the join operator by the semijoin operator. We discuss some interesting connections between the semijoin algebra and the guarded fragment of first-order logic. We also provide an Ehrenfeucht-Fraisse game, characterizing the discerning power of the semijoin algebra. This game gives a method for showing that certain queries are not expressible in the semijoin algebra. 
Dirk Leinders (1), Jerzy Tyszkiewicz (2), Jan Van den Bussche (1) ((1)   Limburgs Universitair Centrum, Diepenbeek, Belgium,...