title: Counting null values between dates
body: <p>I'm trying to calculate the number of null values between dates.</p>
  
  <p>My table looks like this:</p>
  
  <pre><code>transaction_date    transaction_sale
  10/1/2018           NULL
  11/1/2018           33
  12/1/2018           NULL
  1/1/2019            NULL
  2/1/2019            NULL
  3/1/2019            2
  4/1/2019            NULL
  5/1/2019            NULL
  6/1/2019            10
  </code></pre>
  
  <p>I'm looking to get the following output:</p>
  
  <pre><code>transaction_date    transaction_sale   count
  10/1/2018           NULL               NULL
  11/1/2018           33                 1
  12/1/2018           NULL               NULL
  1/1/2019            NULL               NULL
  2/1/2019            NULL               NULL
  3/1/2019            2                  3
  4/1/2019            NULL               NULL
  5/1/2019            NULL               NULL
  6/1/2019            10                 2
  </code></pre>