Test struct for at least one non null value - google-bigquery

Question

Given the data set:
field_a
description_1
metric_1
ball
large
20
ball
small
4
cat
null
null
I want to pack fields description_1 and metric_1 into a struct and then zip them into an array by field_a:
WITH
DATA AS (
SELECT
'ball' AS field_a,
'large' AS description_1,
20 AS metric_1
UNION ALL
SELECT
'ball',
'small',
4
UNION ALL
SELECT
'cat',
NULL,
NULL )
SELECT
field_a,
ARRAY_AGG(STRUCT(description_1,
metric_1))
FROM
DATA
GROUP BY
1;
That gives me:
However, if all fields in the struct are null I would like to see an empty array instead of an array with size of 1 and an struct inside of it.
Desired output:
I figured the following query to test the fields before packing them into the struct:
WITH
DATA AS (
SELECT
'ball' AS field_a,
'large' AS description_1,
20 AS metric_1
UNION ALL
SELECT
'ball' AS field_a,
'small',
4
UNION ALL
SELECT
'cat',
NULL,
NULL )
SELECT
field_a,
ARRAY_AGG(
IF
(description_1 IS NOT NULL
OR metric_1 IS NOT NULL,
STRUCT(description_1,
metric_1),
NULL)IGNORE NULLS)
FROM
DATA
GROUP BY
1;
However it feels like a tedious solution, is there a better way to test if struct contains at least one non-null value? or any other solution to achieve the desired output more elegantly?

Answer 1

Below approach might be slightly cleaner one
select * from (select distinct field_a from your_table)
left join (
select field_a, array_agg(struct(description_1, metric_1))
from your_table
where not description_1 is null and not metric_1 is null
group by field_a
)
using(field_a)
if applied to sample data in your question - output is
Note: there are more ways to write that where clause and depends on your tastes
for example
where format('%t', (description_1, metric_1)) != '(NULL, NULL)'
or
where to_json_string((description_1, metric_1)) != '{"":null,"":null}'
But, I feel, using explicit not ... is ... is more natural and explicit so easier to swallow if you have just two or three columns to check.
When you have more - like 5 or 10, etc. last two version can produce more compact code ...