title: In Sql using UNIQUE
body: <p>I wrote a "Find sids who made exactly one reservation" query with a <code>GROUP BY</code> and <code>HAVING</code> clause like this:</p>
  
  <pre><code>SELECT
    R.sid 
  FROM
    Sailors S,
    Reserve R 
  WHERE  
    R.sid = S.sid 
  GROUP BY 
    R.sid 
  HAVING 
    Count(R.sid) = 1
  </code></pre>
  
  <p>But I want to also find it by using <code>unique</code> command.  My friend asked me but I can't solve it.  We don't use group by.  We solve it with unique.  I tried but i can't solve it.  Can you help me?</p>