The CData Excel Add-In for Jira Assets supports standard SQL joins like the following examples.

Inner Join

An inner join selects only rows from both tables that match the join condition:

 SELECT d.Id, f.ChildValue FROM Department d INNER JOIN Department_CF_ARelCol f ON d.Id = f.ParentId 

Left Join

A left join selects all rows in the FROM table and only matching rows in the JOIN table:

 SELECT d.Id, f.ChildValue FROM Department d LEFT JOIN Department_CF_ARelCol f ON d.Id = f.ParentId