title: cross-dbms way to check if string is numeric
body: <p>Ok, I have this field:  <code>code varchar(255)</code>. It contains some values used in our export routine like</p>
  
  <pre><code>DB84
  DB34
  3567
  3568
  </code></pre>
  
  <p>I need to select only auto-generated (fully numeric) fields </p>
  
  <pre><code>WHERE is_numeric(table.code)
  </code></pre>
  
  <p><code>is_numeric()</code> checks if <code>code</code> field contains only positive digits.</p>
  
  <p>Can you propose anything that will work both under mysql 5.1 and oracle 10g?</p>