title: adding a row for missing data
body: <p>Between a date range 2017-02-01 - 2017-02-10, i'm calculating a running balance.
  I have days where we have missing data, how would I include these missing dates with the previous days balance ?</p>
  
  <p>Example data:</p>
  
  <p><a href="https://i.stack.imgur.com/B1Ikh.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/B1Ikh.jpg" alt="enter image description here"></a></p>
  
  <p>we are missing data for 2017-02-04,2017-02-05 and 2017-02-06, how would i add a row in the query with the previous balance?
  The date range is a parameter, so could change....</p>
  
  <p>Can i use something like the lag function?</p>