title: Working out the percentage of outcomes in a column within a table
body: <p>I am using SQL developer and have a table called table1 which looks like this (but with loads more data): </p>
  
  <pre><code>item_id    seller_id    warranty   postage_class
  -------    ---------    --------   -------------
  14         2            1          2
  17         6            1          1
  14         2            1          1
  14         2            1          2
  14         2            1          1
  14         2            1          2
  </code></pre>
  
  <p>I want to identify the percentage of items sent by first class. </p>
  
  <p>If anyone could help me out that would be amazing! </p>