How to unnest and pivot two columns in BigQuery

Question

Say I have a BQ table containing the following information

| id    | test.name     | test.score    |
|----   |-----------    |------------   |
| 1     | a                 


        

Answer 1

One option could be using conditional aggregation

select id, 
       max(case when test.name='a' then test.score end) as a,
       max(case when test.name='b' then test.score end) as b,
       max(case when test.name='c' then test.score end) as c
from 
(
select a.id, t
from `table` as a,
unnest(test) as t
)A group by id

Answer 2

Below is generic/dynamic way to handle your case

EXECUTE IMMEDIATE (
  SELECT """
  SELECT id, """ || 
    STRING_AGG("""MAX(IF(name = '""" || name || """', score, NULL)) AS """ || name, ', ') 
  || """
  FROM `project.dataset.table` t, t.test
  GROUP BY id
  """
  FROM (
    SELECT DISTINCT name
    FROM `project.dataset.table` t, t.test
    ORDER BY name
  )
);  

If to apply to sample data from your question - output is

Row     id      a       b       c    
1       1       5       7       null     
2       2       8       null    3    

Answer 3

Conditional aggregation is a good approach. If your tables are large, you might find that this has the best performance:

select t.id,
       (select max(tt.score) from unnest(t.score) tt where tt.name = 'a') as a,
       (select max(tt.score) from unnest(t.score) tt where tt.name = 'b') as b,
       (select max(tt.score) from unnest(t.score) tt where tt.name = 'c') as c
from `table` t;

The reason I recommend this is because it avoids the outer aggregation. The unnest() happens without shuffling the data around -- and I have found that this is a big win in terms of performance.