BigQuery: Select entire repeated field with group - google-bigquery

Question

I'm using LegacySQL, but am not strictly limited to it. (though it does have some methods I find useful, "HASH" for example).
Anyhow, the simple task is that I want to group by one top level column, while still keeping the first instance of a nested+repeated set of data alongside.
So, the following "works", and produces nested output:
SELECT
cd,
subarray.*
FROM [magicalfairy.land]
And now I attempt to just grab the entire first subarray (honestly, I don't expect this to work of course)
The following is what doesn't work:
SELECT
cd,
FIRST(subarray.*)
FROM [magicalfairy.land]
GROUP BY cd
Any alternate approaches would be appreciated.
Edit, for data behaviour example.
If Input data was roughly:
[
{
"cd": "something",
"subarray": [
{
"hello": 1,
"world": 1
},
{
"hello": 2,
"world": 2
}
]
},
{
"cd": "something",
"subarray": [
{
"hello": 1,
"world": 1
},
{
"hello": 2,
"world": 2
}
]
}
]
Would expect to get out:
[
{
"cd": "something",
"subarray": [
{
"hello": 1,
"world": 1
},
{
"hello": 2,
"world": 2
}
]
}
]

Answer 1

You'll have a much better time preserving the structure using standard SQL, e.g.:
WITH T AS (
SELECT
cd,
ARRAY<STRUCT<x INT64, y BOOL>>[
STRUCT(off, MOD(off, 2) = 0),
STRUCT(off - 1, false)] AS subarray
FROM UNNEST([1, 2, 1, 2]) AS cd WITH OFFSET off)
SELECT
cd,
ANY_VALUE(subarray) AS subarray
FROM T
GROUP BY cd;
ANY_VALUE will return some value of subarray for each group. If you wanted to concatenate the arrays instead, you could use ARRAY_CONCAT_AGG.
to run this against your table - try below
SELECT
cd,
ANY_VALUE(subarray) AS subarray
FROM `magicalfairy.land`
GROUP BY cd

Answer 2

Try below (BigQuery Standard SQL)
SELECT cd, subarray
FROM (
SELECT cd, subarray,
ROW_NUMBER() OVER(PARTITION BY cd) AS num
FROM `magicalfairy.land`
) WHERE num = 1
This gives you expected result - equivalent of "ANY ARRAY"
This solution can be extended to "FIRST ARRAY" by adding ORDER BY sort_col into OVER() clause - assuming that sort_col defines the logical order