Joining ticket_fields with tickets table via SQL

Hi,

We are currently extracting all the data from Zendesk to our warehouse in BigQuery to do some further analysis and visualization. 

The problem: the ticket_fields has some custom fields column where our agents define the 'Category' of the problem the customer is facing when he contacts us. This can be related to a defined list of things and is basically a drop-down. Unfortunately, we can't find any way to join via SQL this column with our tickets table. 

What do we want: we want to join both tables in our data warehouse so we get the Category for each ticket created, which will allow us to do some meaningful analysis.

Could someone assist?