Create a new column with an array of column names based on condition

Question

I need to create a new column in a Bigquery table. This new column values must be an array of the colour column names based on a condition. My table "Users" looks like:

User	Red	Blue	Green	Yellow	Other columns not related
First	1	1	0	1
Second	0	0	1	0
Third	1	1	0	0

and the expected table or view must be something like this:

User	Red	Blue	Green	Yellow	Array_col
First	1	1	0	1	Red, Blue, Yellow
Second	0	0	1	0	Green
Third	1	1	0	0	Red, Blue

I am relatively new to SQL and Bigquery sintax, I've tried creating a view and also with

ALTER TABLE myproject.mydataset.users`

and I get the column names with this:

SELECT column_names
FROM `myproject.mydataset`.INFORMATION_SCHEMA.COLUMNS
WHERE (table_name = 'users')

But I can't find how to create the array list and make a column with them. I'm crashing against a wall.

Answer 1

Consider below approach

select *, 
  regexp_extract_all(translate(to_json_string(t), '{}"', ''), r'(\w+):1') as Array_col
from mytable t       

if applied to sample data in your question - output is

Answer 2

Maybe this will be ok:

with mytable as (
  select 'First' as user, 1 as Red, 1 as Blue, 0 as Green, 1 as Yellow union all
  select 'Second' as user, 0 as Red, 0 as Blue, 1 as Green, 0 as Yellow union all
  select 'Third' as user, 1 as Red, 1 as Blue, 0 as Green, 0 as Yellow
)
select *, split(trim(if(Red=1, 'Red', '') || if(Blue=1, ' Blue', '') || if(Green=1, ' Green', '') || if(Yellow=1, ' Yellow', ''), ' '), ' ')
from mytable

Answer 3

Now you can try super new syntax for BigQuery! Let's do this with UNPIVOT statement:

WITH mytable AS (
  SELECT 'first' AS user, 1 AS Red, 1 AS Blue, 0 AS Green, 1 AS Yellow UNION ALL
  SELECT 'second' AS user, 0 AS Red, 0 AS Blue, 1 AS Green, 0 AS Yellow UNION ALL
  SELECT 'third' AS user, 1 AS Red, 1 AS Blue, 0 AS Green, 0 AS Yellow
)
SELECT t.*, Array_col FROM (
    SELECT user, ARRAY_AGG(color_name) Array_col FROM mytable 
    UNPIVOT(check_color FOR color_name IN (Red,Blue,Green,Yellow))
    WHERE check_color = 1
    GROUP BY user    
)
JOIN mytable t USING (user) 

Answer 4

Yes, the column names are always the same...

In case if you want to consider only those 4 columns - check below

select t1.*, 
  array(select color from t2.colors where value = 1) Array_col
from mytable t1, 
unnest([struct([struct<value int64, color string>
  (Red, 'Red'), (Blue, 'Blue'), (Green, 'Green'), (Yellow, 'Yellow')
] as colors)]) t2