title: Selecting from relation based on other relation
body: <p>Given a relation A(a,b,c), and a relation B(a,d,e), using projection to isolate 'a' in 'B' like so 'B_=projection_{a}(B)', is there a way to exclude all tupples in 'A', that does not have an 'a' in common with 'B'? </p>
  
  <p>Note that i'm only using relational algebra, and not the extended version.</p>