title: Query the amount of consecutive negative account balance days in BigQuery
body: <p>I have a collection of account balances:</p>
  
  <pre><code>+---------+---------------+---------+------------+
  |    ID   |  customer_id  |  value  | timestamp  |
  +---------+---------------+---------+------------+
  | 1       | 1             |  -200   | 2019-11-18 |
  | 2       | 1             |  100    | 2019-11-17 |
  | 3       | 1             |  -500   | 2019-11-16 |
  | 4       | 1             |  -200   | 2019-11-15 |
  | 5       | 2             |  200    | 2019-11-15 |
  | 6       | 1             |  0      | 2019-11-14 |
  +---------+---------------+---------+------------+
  </code></pre>
  
  <p>I want to get the amount of consecutive days where the customer had a negative account balance, since that last positive account balance. The result should look like that:</p>
  
  <pre><code>+---------------+---------------------------------+------------+
  |  customer_id  |  Negative account balance since |    Date    |
  +---------------+---------------------------------+------------+
  | 1             |  1 day                          | 2019-11-18 |
  +---------------+---------------------------------+------------+
  </code></pre>
  
  <p>Customer #1 had a couple of negative days, however the counter kind of restarts because there was a positive day at 2019-11-17. The date column shows the date of the most recent negative account balance record of that customer. Customer #2 is not in the result, because he had no negative days at all. How can I create such query in BQ?</p>