title: In Proc Timeseries, how to output all ids even if there aren't any occurrences?
body: <p>I have a transactional table with events since 1999, which I want to convert into a time series. However, when I run proc timeseries, it only keeps client ids which have occurrences between my start and end dates. How can I keep all ids, filling those with no occurrences in the period with zeroes?</p>
  
  <p>Example warning message:</p>
  
  <pre><code>WARNING: There are no usable input observations.
  NOTE: The above message was for the following BY group:
        id=1
  </code></pre>
  
  <p>I actually solved this issue in an ugly way, but I'm hoping there's a better one:</p>
  
  <ol>
  <li>Find out which ids were discarded in the time series</li>
  <li>Find out the intervals created by proc timeseries</li>
  <li>Manual cartesian product (discarded_ids x intervals)</li>
  <li>Append</li>
  </ol>