title: How to check if more than one column is empty or NULL
body: <p>I'm trying to write a SELECT statement that will return all rows where two columns are either empty or NULL. Both columns need to be blank or NULL. </p>
  
  <p>I've tried this query within PHPMyAdmin - however, it's still returning some rows where the column value for either is 'on'. </p>
  
  <pre><code>SELECT * 
          FROM product
          WHERE isstock IS NULL 
                     OR isstock = ''
          AND
          iscon IS NULL 
                     OR iscon = ''
  </code></pre>
  
  <p>Can anybody explain where I'm going wrong? </p>
  
  <p>Thanks,</p>