title: SQL: How do I SELECT only the rows with a unique value on certain column?
body: <p>Thanks a million everyone for everyone's response.  Unfortunately, none of the solutions appear to be working on my end, and my guess is that the example I've provided is messed up.</p>
  
  <p>So let me try again.</p>
  
  <p>My table looks like this:</p>
  
  <pre><code>    contract    project activity
  row1    1000    8000    10
  row2    1000    8000    20
  row3    1000    8001    10
  row4    2000    9000    49
  row5    2000    9001    49
  row6    3000    9000    79
  row7    3000    9000    78
  </code></pre>
  
  <p>Basically, the query I'm looking for would return "2000,49" for "contract, activity" because only contract #2000 has one, and ONLY one, unique activity value.</p>
  
  <p>Again, thanks a million in advance,
  boroatel</p>