title: Group only the adjacent rows
body: <p>hi i have a table like this:</p>
  
  <pre><code>notifies
  
  id,user_id
  1,3
  2,3
  3,4
  4,5
  5,6
  6,3
  7,4
  8,4
  9,3
  10,3
  </code></pre>
  
  <p>I have to create a query that group only the adjacent rows </p>
  
  <p>So, the result of this example should be:</p>
  
  <pre><code>user_id
  3
  4
  5
  6
  3
  4
  3
  </code></pre>
  
  <p>How can i do that?
  thanks</p>