title: Finding missing dates in a sequence
body: <p>I have following table with ID and DATE</p>
  
  <pre><code>ID   DATE
  123   7/1/2015
  123   6/1/2015
  123   5/1/2015
  123   4/1/2015
  123   9/1/2014
  123   8/1/2014
  123   7/1/2014
  123   6/1/2014
  456   11/1/2014
  456   10/1/2014
  456   9/1/2014
  456   8/1/2014
  456   5/1/2014
  456   4/1/2014
  456   3/1/2014
  789   9/1/2014
  789   8/1/2014
  789   7/1/2014
  789   6/1/2014
  789   5/1/2014
  789   4/1/2014
  789   3/1/2014
  </code></pre>
  
  <p>In this table, I have three customer ids, 123, 456, 789 and date column which shows which month they worked.</p>
  
  <p>I want to find out which of the customers have gap in their work.
  Our customers work record is kept per month...so, dates are monthly..
  and each customer have different start and end dates.</p>
  
  <p>Expected results:</p>
  
  <pre><code>ID     First_Absent_date
  
  123    10/01/2014
  456    06/01/2014
  </code></pre>