title: Aggregate functions in selection clause in relational algebra
body: <p>Is it possible to compare an attribute value to the value of an aggregate function in relational algebra? I would like to be able to say something like the following:</p>
  
  <p>(SELECTION) clause= attribute1 > count(attribute2)</p>
  
  <p>My teacher is sort of a stickler on only using methods described in class, however, if it is an established part of relational algebra I can argue him into accepting my answer as he is a mathematician first and fore most and couldn't take me stating he is restricting established parts of a system.</p>