title: SQL Last 12 months sum
body: <p>Need sql help from the below data .
  <code>ExpectedResult</code> is the column I would like to derive .</p>
  <pre><code>MonthBeginDate  cnt ExpectedResult
  1/12/2019   51  0
  1/01/2020   161 0
  1/02/2020   243 0
  1/03/2020   252 0
  1/04/2020   214 0
  1/05/2020   309 0
  1/06/2020   309 0
  1/07/2020   324 0
  1/08/2020   363 0
  1/09/2020   311 0
  1/10/2020   373 0
  1/11/2020   260 0
  1/12/2020   249 3419
  1/01/2021   205 3573
  1/02/2021   255 3667
  1/03/2021   217 3641
  1/04/2021   165 3554
  1/05/2021   158 3498
  1/06/2021   158 3347
  1/07/2021   237 3275
  1/08/2021   319 3270
  1/09/2021   227 3134
  1/10/2021   297 3120
  1/11/2021   16  2763
  </code></pre>