Calculate a running total with a condition in BigQuery

Sorry bad topic… I need to calculate a running total but need to reset the total on a condition (when expected reached = 0). I have this table:

Date, Registrations, Expected Registrations, Expected reached
        2020-03-01, 5, 4,1
        2020-03-02, 7, 5,1
        2020-03-03, 8, 6,1
        2020-03-04, 2, 5,0
        2020-03-05, 5, 4,1
        2020-03-06, 7, 5,1
        2020-03-07, 8, 6,1
        2020-03-08, 2, 5,0

Expected result with running total – the condition is that while “Expected Reached” <> 0 running total should be calculated. If “Expected Reached” = 0 the running total should start over from 0:

Date, Registrations, Expected Registrations, Expected Reached, Running Total
            2020-03-01, 5, 4,1, 1
            2020-03-02, 7, 5,1, 2
            2020-03-03, 8, 6,1, 3
            2020-03-04, 2, 5,0, 0
            2020-03-05, 5, 4,1, 1
            2020-03-06, 7, 5,1, 2
            2020-03-07, 8, 6,1, 3 
            2020-03-08, 2, 5,0, 0

I don’t know how to partition my window function to do this. Maybe I have to create an intermediate calculation before but Im unsure. Any suggestions?

Edit: edit2: removed my “on the fly question”.

Answer

Below is for BigQuery Standard SQL

#standardSQL
SELECT * EXCEPT(grp), 
  SUM(Expected_reached) OVER(PARTITION BY grp ORDER BY `date`) Running_Total
FROM (
  SELECT *, COUNTIF(Expected_reached = 0) OVER(ORDER BY `date`) grp 
  FROM `project.dataset.table`
)