title: MySQL - Updating rows that have different (not unique) value in one column
body: <p>I have table with items. Each item has id, item_type (let's say - book, audio, video, ebook), time of insertion and sign:</p>
  
  <pre><code>id|  item_type | created (timestamp) | sign
  1 |    book    | 2014-01-14 15:40:24 | NULL
  2 |    book    | 2014-01-15 15:40:24 | NULL
  3 |   audio    | 2014-01-16 15:40:24 | NULL
  4 |   audio    | 2014-01-17 15:40:24 | NULL
  5 |   ebook    | 2014-01-18 15:40:24 | NULL
  6 |   video    | 2014-01-19 15:40:24 | NULL
  </code></pre>
  
  <p>I would like to update column sign for 3 oldest rows with different item_type, for data above, these rows are with ID 1,3,5.</p>
  
  <p>But I have no idea how to express the condition that item_type must be different for updated rows. Thank you for your advices. I appreciate your help. Richard</p>
  
  <p>EDIT: apologies if description is confusing: 
  I would like to update <strong>always 3 rows</strong>, each row with <strong>different</strong> item_type. Not 3 books, 3 videos, ...</p>
  
  <p><br>Ad sorting - it's true I could sort them by ID, not timestamp here. This demo is simplification, in real application, I will use use sorting by modification timestamp.</p>