How to unpivot in BigQuery?

Not sure what functions to call, but transpose is the closest thing I can think of.

I have a table in BigQuery that is configured like this:

but I want to query a table that is configured like this:

What does the SQL code look like for creating this table?

Thanks!

2020 update: fhoffa.x.unpivot()

See:

• https://medium.com/@hoffa/how-to-unpivot-multiple-columns-into-tidy-pairs-with-sql-and-bigquery-d9d0e74ce675

I created a public persistent UDF. If you have a table a, you can give the whole row to the UDF for it to be unpivotted:

SELECT geo_type, region, transportation_type, unpivotted
FROM `fh-bigquery.public_dump.applemobilitytrends_20200414` a
  , UNNEST(fhoffa.x.unpivot(a, '_2020')) unpivotted

It transforms a table like this:

Into this

---

As a comment mentions, my solution above doesn't solve for the question problem.

So here's a variation, while I look how to integrate all into one:

CREATE TEMP FUNCTION unpivot(x ANY TYPE) AS (
(
  SELECT 
   ARRAY_AGG(STRUCT(
     REGEXP_EXTRACT(y, '[^"]+') AS key
   , REGEXP_EXTRACT(y, ':([0-9]+)') AS value
   ))
  FROM UNNEST((
    SELECT REGEXP_EXTRACT_ALL(json,'"[smlx][meaxl]'||r'[^:]+:\"?[^"]+?') arr
    FROM (SELECT TO_JSON_STRING(x) json))) y
)
);

SELECT location, unpivotted.*
FROM `robotic-charmer-726.bl_test_data.reconfiguring_a_table` x
  , UNNEST(unpivot(x)) unpivotted

---

Previous answer:

Use the UNION of tables (with ',' in BigQuery), plus some column aliasing:

SELECT Location, Size, Quantity
FROM (
  SELECT Location, 'Small' as Size, Small as Quantity FROM [table]
), (
  SELECT Location, 'Medium' as Size, Medium as Quantity FROM [table]
), (
  SELECT Location, 'Large' as Size, Large as Quantity FROM [table]
)