How to create a new table with nested data in big query from another tables? - google-cloud-platform

Question

I found lots of example to create nested data in google bigquery manual but there is no example to do this from another tables.
I want to create a new table (for example solar_system_moons_nested) with nested data (write SQL statement to generate the nested data) using two existing tables (for example planets and moons tables). I want the new table look as follows:
I create the moon and planet tables as below:
moon table
planet table:
Is there anyway to create a nested table from existing tables? any help would be appreciated.
Here is how I made the new table(as below):
WITH solar_system_moons_nested AS (
SELECT p.planet,
STRUCT(moon ,Distance_from_Planet__km_,Diameter__km_) AS moons,
from test.planets p inner join test.moons m on m.planet=p.planet
)
select * from solar_system_moons_nested
and here is how it look like:
As you see, the select did not do what I expected.

Answer 1

If all you want is a nested structure, you can use array_agg and do something like below
WITH solar_system_moons_nested AS (
SELECT p.planet,
ARRAY_AGG(STRUCT(moon ,Distance_from_Planet__km_,Diameter__km_)) AS moons,
from test.planets p inner join test.moons m on m.planet=p.planet
GROUP BY 1
)
select * from solar_system_moons_nested
More on array_agg here https://cloud.google.com/bigquery/docs/reference/standard-sql/aggregate_functions

Answer 2

Use array_agg to construct an array:
WITH solar_system_moons_nested AS (
SELECT
p.planet,
array_agg(STRUCT(moon ,Distance_from_Planet__km_,Diameter__km_)) AS moons,
from test.planets p inner join test.moons m on m.planet=p.planet
group by p.planet
)
select * from solar_system_moons_nested