How do I group by into struct in BigQuery? - google-cloud-platform

Question

I have the following data:
player_id level talent_id
1 1 a
1 2 b
1 3 c
2 1 d
2 2 e
And want to group by player_id and have rows as structs, with level values as struct field names:
player_id data
1 {_1 = a, _2 = b, _3 = c}
2 {_1 = d, _2 = e, _3 = null}
the level column is always from the set of {1, 2, 3} but some levels might be missing (null)
What I've got so far is aggregation by player_id and with attached array of results:
talents as (
select
p.player_id,
array_agg(struct(p.level, p.talent_id)) as talents
from source.player_talent p
group by player_id
),
player_id data
1 [{1, a}, {2, b}, {3, c}]
2 {{1, d}, {2, e}]
now I need to map this array to a struct with fixed property names _1, _2, _3

Answer 1

This returns the expected results:
WITH Players AS (
SELECT 1 AS player_id, 1 AS level, 'a' AS talent_id UNION ALL
SELECT 1, 2, 'b' UNION ALL
SELECT 1, 3, 'c' UNION ALL
SELECT 2, 1, 'd' UNION ALL
SELECT 2, 2, 'e'
)
SELECT
player_id,
STRUCT(
MAX(IF(level = 1, talent_id, NULL)) AS _1,
MAX(IF(level = 2, talent_id, NULL)) AS _2,
MAX(IF(level = 3, talent_id, NULL)) AS _3) AS data
FROM Players
GROUP BY player_id
The technique here is known as pivoting (converting rows to columns).