How do I output a set of columns and values, with one column per row as key value pairs in presto?

Question

I have data of the form

id | col1 | col2 | col3 | col4 | col5 | col6 |
----------------------------------------------
1 |   a   |    b |   c  |   d  |   e  |   f  |
2 |   a   |    b |   c  |   d  |   e  |   f  |
3 |   a   |    b |   c  |   d  |   e  |   f  |

that I'm trying to get into the form

id | key | value |
------------------
1  | col1| a
1  | col2| b
1  | col3| c
1  | col4| d
1  | col5| e
1  | col6| f
2  | col1| a
2  | col2| b
2  | col3| c
2  | col4| d
2  | col5| e
2  | col6| f
3  | col1| a
3  | col2| b
3  | col3| c
3  | col4| d
3  | col5| e
3  | col6| f

and I can't for the life of me figure out how to go about doing it. I can accomplish the opposite and turn a map into a single row based on a key via doing something like the follows, but I'm not sure how to go from a single row to many rows based on the columns.

SELECT
  id,
  key['a'] AS col1,
  key['b'] AS col2
FROM (
  SELECT id, map_agg(key, value) key
  FROM table_a
  GROUP BY id
) temp

Is this something that is possible in presto?

Answer 1

I don't have Presto/Athena onhand to test this, but I think the approach is:

select t.id, kv[1] as key, kv[2] as value
from (select t.*,
             array[row('col1', col1),
                   row('col2', col2),
                   row('col3', col3),
                   row('col4', col4),
                   row('col5', col5),
                   row('col6', col6)
                  ] as kv_ar
      from t
     ) t cross join
     unnest(kv_ar) kv

Answer 2

You can zip arrays & unnest them using cross join unnest . so, construct a zipped array using column names & column values and then use unnest.

with test (id,col1,col2,col3,col4,col5,col6) AS (
values
(1,'a','b','c','d','e','f'),
(2,'a','b','c','d','e','f'),
(3,'a','b','c','d','e','f')
)
select id, k, v
from test
cross join unnest(
    array['col1', 'col2', 'col3', 'col4','col5', 'col6']
  , array[col1, col2, col3, col4, col5, col6]
) as x(k, v)