title: Get a unique count of users over a rolling 3 month period
body: <p>So I have a table that has a column for usernames and then a column with the 1st of the month for each time that user logged into our service. I would like to get a unique user count looking a rolling three months backward for every month in the table. So for example the data looks like this:</p>
  
  <pre><code>username  Month
  Bob       01/01/2017
  Joe       12/01/2017
  Sam       01/01/2017
  Bob       01/01/2017
  Joe       12/01/2017
  Frank     02/01/2017
  Alice     02/01/2017
  John      02/01/2017
  Jen       03/01/2017
  </code></pre>
  
  <p>and I would want my results to look like this:</p>
  
  <pre><code>Month         Rolling3MonthUniqueUserCount
  12/01/2017    1
  01/01/2017    3
  02/01/2017    6
  03/01/2017    6
  </code></pre>
  
  <p>The query (or Multiple queries) have to do two things:</p>
  
  <ol>
  <li>identify the day 3 months back from the date in
  the table</li>
  <li>Get the unique user count within that 3 month period</li>
  </ol>
  
  <p>For the life of me I cant figure out how to do this, and any help would be appreciated. Thanks :) </p>
  
  <p>P.S. Im using access which uses T-SQL. Which slightly limits the functions that you can use. </p>