Question: 1. What domains are the catalog relations themselves defined on?
2. A relation is defined to have a set of attributes and a set of tuples. Now, in mathematics the empty set is a perfectly respectable set; indeed, it is usually desirable that results, theorems, etc., that hold true for a set of n elements should continue to hold true if n= 0. Can a relation have an empty set of tuples? Or an empty set of attributes?