title: SQL How to create where clause to filter last six periods with this format YYYYMM?
body: <p>I want to run a query with a criteria that sums the previous 6 periods with the following period format <code>YYYYMM</code>.</p>
  
  <pre><code>SELECT sum(t.usage)
  FROM t
  WHERE t.peroid ?????
  GROUP BY t.period
  
  Period   Usage
  -------------
  201907   30
  201908   40
  201909   50
  201910   60
  201911   70
  201912   60
  202001   20
  </code></pre>