How to unnest bigquery field that is stored as a string? - google-bigquery

Question

I am trying to unnest a field but something is wrong with my query.
Sample data in my table
'1234', '{ "id" : "123" , "items" : [ { "quantity" : 1 , "product" : { "id" : "p1" , "categories" : [ "cat1","cat2","cat3"] }}] }'
There are 2 fields in the dataset: row_id and parts, where parts is a dictionary object with list items (categories) in it but datatype of a parts is string. I would like the output to be individual row for each category.
This is what I have tried but I am not getting any result back.
#standardSQL
with t as (
select "1234" as row_id, '{ "id" : "123" , "items" : [ { "quantity" : 1 , "product" : { "id" : "p1" , "categories" : [ "cat1","cat2","cat3"] }}] }' as parts
)
select row_id, _categories
from t,
UNNEST(REGEXP_EXTRACT_ALL(JSON_EXTRACT(parts, '$.items'), r'"categories":"(.+?)"')) _categories
expected result
id, _categories
1234, cat1
1234, cat2
1234, cat3

Answer 1

Below is for BigQuery Standard SQL
#standardSQL
WITH t AS (
SELECT "1234" AS row_id, '{ "id" : "123" , "items" : [ { "quantity" : 1 , "product" : { "id" : "p1" , "categories" : [ "cat1","cat2","cat3"] }}] }' AS parts
)
SELECT row_id, REPLACE(_categories, '"', '') _categories
FROM t, UNNEST(SPLIT(REGEXP_EXTRACT(
JSON_EXTRACT(parts, '$.items'),
r'"categories":\[(.+?)]'))
) _categories
and produces expected result
Row row_id _categories
1 1234 cat1
2 1234 cat2
3 1234 cat3
Update
Above solution was mostly focused on fixing regexp used in extract - but not addressed more generic case of having multiple products. Below solution addresses such more generic case
#standardSQL
WITH t AS (
SELECT "1234" AS row_id, '''{ "id" : "123" , "items" : [
{ "quantity" : 1 , "product" : { "id" : "p1" , "categories" : [ "cat1","cat2","cat3"] }},
{ "quantity" : 2 , "product" : { "id" : "p2" , "categories" : [ "cat4","cat5","cat6"] }}
] }''' AS parts
)
SELECT row_id, REPLACE(category, '"', '') category
FROM t, UNNEST(REGEXP_EXTRACT_ALL(parts, r'"categories" : \[(.+?)]')) categories,
UNNEST(SPLIT(categories)) category
with result
Row row_id category
1 1234 cat1
2 1234 cat2
3 1234 cat3
4 1234 cat4
5 1234 cat5
6 1234 cat6