How to unpivot in BigQuery?

Question

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!

Answer 1

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]
)

Answer 2

@Felipe, I tried this using standard-sql but I get an error on the first line of your query that says: "Column name Location is ambiguous at [1:8]"

I've used an alternate query that works for me:

SELECT Location, 'Small' as Size, Small as Quantity FROM `table`
UNION ALL
SELECT Location, 'Medium' as Size, Medium as Quantity FROM `table`
UNION ALL
SELECT Location, 'Large' as Size, Large as Quantity FROM `table`

Answer 3

I have a solution that uses STRUCTs, ARRAYs and CROSS JOIN + UNNEST:

WITH
  My_Table_Metrics_Data AS (
  SELECT
    ...,
    [
        STRUCT('...' AS Metric, ... AS Data),
        STRUCT('...' AS Metric, ... AS Data),
    ] AS Metrics_Data
  FROM
    `My_Dataset.My_Table`
  WHERE
    ...
  )
SELECT
  ...,
  Metric_Data
FROM
  My_Table_Metrics_Data
CROSS JOIN
  UNNEST(My_Table_Metrics_Data.Metrics_Data) AS Metric_Data

Full explanation and instructions: https://yuhuisdatascienceblog.blogspot.com/2018/06/how-to-unpivot-table-in-bigquery.html