The CData JDBC Driver for Jira 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 Projects.LeadName, Issues.Summary FROM Projects, Issues WHERE Projects.Id=Issues.ProjectId

Left Join

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

SELECT Projects.LeadName, Issues.Summary FROM Projects LEFT OUTER JOIN Issues ON Projects.Id=Issues.ProjectId