Programmatic way to loop over columns and replace null values with zeros in BigQuery?

Question

I am trying to prepare a large data.table in BigQuery for a regression that involves lots of "dummy" (aka categorical) variables.

One of final steps in this process requires me to effectively replace all instances of null values in the table with zeros.

Is there a clean and programmatic way to do this in Big Query? For example, in the table below, I'd ideally like to loop over all the "country_*" fields, and replace with zero in a non hard coded fashion. I have an inkling that this may be a job for dynamic SQL, but I get pretty lost swimming in the documentation. Any help would be greatly appreciated!

TLDR: This is an example of the data structure I'm facing.

country	country_1	country_2	country_3	other covariates
1	1	-	-
2	-	1	-
3	-	-	1

This is what I'd like to have

country	country_1	country_2	country_3	other covariates
1	1	0	0
2	0	1	0
3	0	0	1

Simpleton method:

select country, 
       ifnull(country_1, 0) as country_1,
       ...
FROM TABLE

Answer 1

Try below

create temp function  extract_keys(input string) returns array<string> language js as "return Object.keys(JSON.parse(input));";
create temp function  extract_values(input string) returns array<string> language js as "return Object.values(JSON.parse(input));";
select * except(json)
from (
  select json, col, val
  from your_table t,
  unnest([struct(replace(to_json_string(t), ':null', ':0') as json)]),
  unnest(extract_keys(json)) col with offset
  join unnest(extract_values(json)) val with offset
  using(offset)
)
pivot (any_value(val) for col in ('country', 'country_1', 'country_2', 'country_3'))    

if applied to sample data in your question - output is