Finding missing dates in a sequence
<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>