title: Check for != values when column contains records with NULLs
body: <p>I have a number of tables that mix 'real' values with nulls in columns. From exerience, issuing a SELECT against these that looks like:</p>
  
  <pre><code>SELECT column1, column2, column3 FROM mytable WHERE column1 != 'a value';
  </code></pre>
  
  <p>...doesn't return the records I expect. In the current table I am working on, this returns an empty recordset, even though I know I have records in the table with NULLs in column1, and other records in the table that have the value I am "!="ing in column1. I am expecting, in this case, to see the records with NULLs in column1 (and, of course, anything else if there were other not 'a value' values in column1.</p>
  
  <p>Experimenting with NVL in the WHERE clause doesn't seem to give me anything different:</p>
  
  <pre><code>SELECT column1, column2, column3 FROM mytable WHERE NVL(column1, '') != 'a value';
  </code></pre>
  
  <p>...is also returning an empty recordset.</p>
  
  <p>Using 'IS NULL' will technically give me the correct recordset in my current example, but of course if any records change to something like 'another value' in column1, then IS NULL will exclude those.</p>