 <p>There are exactly two relations with no attributes, one with an empty tuple, and one without. In <a href="http://en.wikipedia.org/wiki/The_Third_Manifesto" rel="nofollow noreferrer">The Third Manifesto</a>, Date and Darwen (somewhat) humorously name them <code>TABLE_DEE</code> and <code>TABLE_DUM</code> (respectively).</p> <p>They are useful to the extent that they are the identity of a variety of relational operators, playing roles equivalent to 1 and 0 in ordinary algebra.</p>