BigQuery Standard SQL Pivot Structs and Sum over non-overlapping windows - google-bigquery

Question

I'm trying to query a table that uses a basic repeating field to store data like this:
+---+----------+------------+
| i | data.key | data.value |
+---+----------+------------+
| 0 | a | 1 |
| | b | 2 |
| 1 | a | 3 |
| | b | 4 |
| 2 | a | 5 |
| | b | 6 |
| 3 | a | 7 |
| | b | 8 |
+---+----------+------------+
I'm trying to figure out how to run a query that gets a result like
+---+----+----+
| i | a | b |
+---+----+----+
| 1 | 4 | 6 |
| 3 | 12 | 14 |
+---+----+----+
where each row represents a non-overlapping sum (i.e. i=1 is the sum of rows i=0 and i=1) and the data has been pivoted such that data.key is now a column.
Problem 1:
I did my best to convert this answer to use Standard SQL and ended up with:
SELECT
i,
(SELECT SUM(value) FROM UNNEST(data) WHERE key = 'a') as `a`,
(SELECT SUM(value) FROM UNNEST(data) WHERE key = 'b') as `b`
FROM
`dataset.testing.dummy`)
This works, but I'm wondering if there is a better way to do this, especially since it produces a particularly verbose query when trying to use analytic functions:
SELECT
i,
SUM(a) OVER (ORDER BY i ROWS BETWEEN 1 PRECEDING AND CURRENT ROW) AS `a`,
SUM(b) OVER (ORDER BY i ROWS BETWEEN 1 PRECEDING AND CURRENT ROW) AS `b`
FROM (
SELECT
i,
(SELECT SUM(value) FROM UNNEST(data) WHERE key = 'a') as `a`,
(SELECT SUM(value) FROM UNNEST(data) WHERE key = 'b') as `b`
FROM
`dataset.testing.dummy`)
ORDER BY
i;
Problem 2:
How do I write a ROW or RANGE statement such that resulting windows don't overlap. In the last query, I get a rolling sum over the data, which isn't quite what I'm looking to do.
+---+----+----+
| i | a | b |
+---+----+----+
| 0 | 1 | 2 |
| 1 | 4 | 6 |
| 2 | 8 | 10 |
| 3 | 12 | 14 |
+---+----+----+
The rolling sum produces a result for each row, whereas I'm attempting to reduce the number of rows returned.

Answer 1

Using a temporary SQL function plus a named window helps with the verbosity. I had to use another subselect to apply the filter on i afterward, though. Here's a self-contained example:
#standardSQL
CREATE TEMP FUNCTION SumKey(
data ARRAY<STRUCT<key STRING, value INT64>>,
target_key STRING) AS (
(SELECT SUM(value) FROM UNNEST(data) WHERE key = target_key)
);
WITH Input AS (
SELECT
0 AS i,
ARRAY<STRUCT<key STRING, value INT64>>[('a', 1), ('b', 2)] AS data UNION ALL
SELECT 1, ARRAY<STRUCT<key STRING, value INT64>>[('a', 3), ('b', 4)] UNION ALL
SELECT 2, ARRAY<STRUCT<key STRING, value INT64>>[('a', 5), ('b', 6)] UNION ALL
SELECT 3, ARRAY<STRUCT<key STRING, value INT64>>[('a', 7), ('b', 8)]
)
SELECT * FROM (
SELECT
i,
SUM(a) OVER W AS a,
SUM(b) OVER W AS b
FROM (
SELECT
i,
SumKey(data, 'a') AS a,
SumKey(data, 'b') AS b
FROM Input
)
WINDOW W AS (ORDER BY i ROWS BETWEEN 1 PRECEDING AND CURRENT ROW)
)
WHERE MOD(i, 2) = 1
ORDER BY i;
This results in:
+---+----+----+
| i | a | b |
+---+----+----+
| 1 | 4 | 6 |
| 3 | 12 | 14 |
+---+----+----+