title: Running Sum for the last 30 days on BigQuery
body: <p>I am trying to get the following query on Google Merchandise Store public dataset in BigQuery:</p>
  
  <blockquote>
    <ol>
    <li>Date</li>
    <li>Number of distinct users</li>
    <li>Running sum of the number of distinct users in the last 30 days</li>
    </ol>
  </blockquote>
  
  <p>For eg (I used 3 days in the example for simplicity):</p>
  
  <pre><code>  date        distinct_users     distinct_users_3days
  15/07/2018          8                    15
  14/07/2018          2                    12
  13/07/2018          5                    20
  12/07/2018          5                    15
  11/07/2018         10                    10
     ...
  </code></pre>
  
  <p>This is my current SQL code which gets the first two columns, but I can't figure out how to get the running sum:</p>
  
  <pre><code>SELECT
    date,
    COUNT(DISTINCT(fullVisitorId)) as daily_active_user
  FROM
    `bigquery-public-data.google_analytics_sample.ga_sessions_2017*`
  WHERE
    _table_suffix BETWEEN "0101"
    AND "0715"
  GROUP BY
    date
  </code></pre>
  
  <p>Any help is appreciated! :)</p>