title: How to select the rows of a dataframe where two variables have value
body: <p>So I have a dataframe: df1. And I want to select the rows based on the value of two variables != NULL.</p>
  
  <p>For example:</p>
  
  <pre><code>Variable1    Variable2
  
  NULL         5
  
  6            NULL
  
  NULL         NULL
  
  3            6
  
  1            9
  </code></pre>
  
  <p>Result:</p>
  
  <pre><code>Variable1   Variable2
  
  3            6
  
  1            9
  </code></pre>