The CData JDBC Driver for Jira Service Desk 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 c.RequestTypeId, o.Name FROM Requests c INNER JOIN RequestsParticipants o ON c.RequestId = o.RequestId 

Left Join

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

 SELECT c.RequestTypeId, o.Name FROM Requests c LEFT JOIN RequestsParticipants o ON c.RequestId = o.RequestId