title: Filling in missing balance and dates in table to track balance
body: <p>I hope you can help me with this problem. I just started out on SQL using Bigquery so my problem can seem a bit tedious.
  So I have a table that basically records the date and balance whenever the balance changes. It looks somewhat like this:</p>
  <pre><code>
  +------------+-----------+------+---------+
  |    Date    | seller_ID | Name | Balance |
  +------------+-----------+------+---------+
  | 2020-09-10 |         1 | John |    10   |
  | 2020-09-13 |         1 | John |    8    |
  | 2020-09-15 |         1 | John |    6    |
  +------------+-----------+------+---------+
  </code></pre>
  <p>However, I need to create a new table with the daily balances that looks like this</p>
  <pre><code>+------------+-----------+------+---------+
  |    Date    | seller_ID | Name | Balance |
  +------------+-----------+------+---------+
  | 2020-09-10 |         1 | John |      10 |
  | 2020-09-11 |         1 | John |      10 |
  | 2020-09-12 |         1 | John |      10 |
  | 2020-09-13 |         1 | John |       8 |
  | 2020-09-14 |         1 | John |       8 |
  | 2020-09-15 |         1 | John |       6 |
  +------------+-----------+------+---------+
  
  </code></pre>
  <p>I tried creating a separate table of all the dates between the first and final date, and then LEFT JOIN the original table with it but the resulting table isn't very helpful to draw from.
  Does anyone have an idea of what to do in this case?</p>