title: How to select for observations that are null for one variable and not null for another?
body: <p>I need to select observations from a dataset in a way that excludes rows missing certain values, but having others.  For example, I have the following variables: </p>
  
  <p>A, dateA, B, dateB, C, dateC</p>
  
  <p>I need to exclude rows that are not null for A, but are missing a dateA value OR are not null for B, but are missing a dateB value OR are not null for C but are missing a dateC value. Also, because my variables are structured so that they can be empty values, I have been using the following to code to identify "null" observations:</p>
  
  <pre><code>WHERE dateA &gt; '';
  </code></pre>
  
  <p>I'm not sure what to put in my select statement to accomplish this.  Any help would be much appreciated! Thanks!</p>
  
  <p>EDIT here would be some example data: (sorry for the awkward formatting) </p>
  
  <p><strong>Obs</strong>----<strong>A</strong>-----<strong>dateA</strong>        </p>
  
  <p>1--------3-----2002-02-13 00:00:00</p>
  
  <p>2--------8-----2009-12-19 00:00:00</p>
  
  <p>3--------0</p>
  
  <p>4--------7</p>
  
  <p>The result I would want would be to select observations 1, 2, and 3, but not 4 because it is missing a value for dateA, but has a value for A. 3 should be included because although it is missing a value for dateA, it has a 0 value for A. I also have other variables, B, dateB, C and dateC that need to be selected for in the same way.  </p>