Creating a KSQL Stream: How to extract value from complex json - json

Question

I am trying to create a stream in Apache/KAFKA KSQL
The topic contains (somewhat complex JSON)
{
"agreement_id": "dd8afdbe-59cf-4272-b640-b14a24d8234c",
"created_at": "2018-02-17 16:00:00.000Z",
"id": "6db276a8-2efe-4495-9908-4d3fc4cc16fa",
"event_type": "data",
"total_charged_amount": {
"tax_free_amount": null,
"tax_amounts": [],
"tax_included_amount": {
"amount": 0.0241,
"currency": "EUR"
}
}
"used_service_units": [
{
"amount": 2412739,
"currency": null,
"unit_of_measure": "bytes"
}
]
}
Now creating a stream is easy for just simple stuff like event_type and created_at. That would be like this
CREATE STREAM tstream (event_type varchar, created_at varchar) WITH (kafka_topic='usage_events', value_format='json');
But now I need to access the used_service_units....
and I would like to extract the "amount" in the JSON above
How would I do this ?
CREATE STREAM usage (event_type varchar,create_at varchar, used_service_units[0].amount int) WITH (kafka_topic='usage_events', value_format='json');
Results in
line 1:78: mismatched input '[' expecting {'ADD', 'APPROXIMATE', ...
And if I instead create a stream like so
CREATE STREAM usage (event_type varchar,create_at varchar, used_service_units varchar) WITH (kafka_topic='usage_events', value_format='json');
And then does a SQL SELECT on the stream like this
SELECT EXTRACTJSONFIELD(used_service_units,'$.amount') FROM usage;
SELECT EXTRACTJSONFIELD(used_service_units[0],'$.amount') FROM usage;
SELECT EXTRACTJSONFIELD(used_service_units,'$[0].amount') FROM usage;
Neither of these alternatives work...
This one gave me
SELECT EXTRACTJSONFIELD(used_service_units[0],'$.amount') FROM usage;'
Code generation failed for SelectValueMapper

Answer 1

It seems that ONE solution to this problem is to make the column datatype an array
i.e.
CREATE STREAM usage (event_type varchar,created_at varchar, total_charged_amount varchar, used_service_units array<varchar> ) WITH (kafka_topic='usage_events', value_format='json');
Now I am able to do the following:
SELECT EXTRACTJSONFIELD(used_service_units[0],'$.amount') FROM usage