This is the query that fetches fields from service desk entities and the relations between them. It's for educational use, to help you familiarize with the structure of the sd database. If you don't understand the structure you won't be able to create reports.
The tables itsm_incidents (inc) and itsm_configuration_items (cit) are joined on the fields inc.inc_cit_oid=cit.cit_oid
You must insert these two tables on the report, specify the link between them (left outer join, in case that an incident is without a ci). On the report insert what BUILD_IN fields of CI you like. If you want to also fetch custom fields of the CI, then you must instert the table "itsm_cit_custom_fields" (ccf) and the join is cit.cit_oid=ccf.ccf_cit_oid.
The other thing that may help is if you have the system admins generate views using the admin panel. The system can generate a number of views (using analyzed data, I think) that would make reporting a lot simpler, as the views contains the data as it appears in the call so you would not need to build joins.
The disadvantage is that you canot control the fields which are in the views so if you need an additional column that is not in the view, you would be back to square 1.