title: Big query sql Using lag function with missing days
body: <p>I am using sql in Big Query superset to create a pivot table to calculate the difference of a game between a range of days:</p>
  <pre><code>Game    Date        Sum     Prev_sum
  First   2022-08-15  178.68  
          2022-08-16  345.09  166.41
          2022-08-18  570.53  991.89   --wrong input ,missing day
          2022-08-19  173.69  -396.84
          2022-08-20  532.35  358.66
          2022-08-21  248.35  -284
  Second  2022-08-17  518.59  600.53   --wrong input ,missing day
          2022-08-20  182.91  148.3
  Third   2022-08-15  527.35  
          2022-08-17  359.93  426.53
          2022-08-20  539.05  564.57
          2022-08-21  543.74  4.69
  </code></pre>
  <p>i m using</p>
  <pre><code> select Date , SUM(Sum) ,
             SUM(Sum) - 
             lag(SUM(Sum)) over(partition by Game order by min(Date) ASC)
     from my_table
  </code></pre>
  <p>ideally i would like to have the difference of sum between each row regardless of the date shown</p>
  <pre><code>Game    Date        Sum     Prev_sum
  First   2022-08-15  178.68  
          2022-08-16  345.09  166.41
          2022-08-18  570.53  225   
          2022-08-19  173.69  -396.84
          2022-08-20  532.35  358.66
          2022-08-21  248.35  -284
  Second  2022-08-17  518.59  
          2022-08-20  182.91  -336
  </code></pre>