The CData Power BI Connector for Basecamp 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.Name, ToDos.AssigneeName FROM Projects, ToDos WHERE Projects.Id=ToDos.ProjectId

Left Join

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

SELECT Projects.Name, ToDos.AssigneeName FROM Projects LEFT OUTER JOIN ToDos ON Projects.Id=ToDos.ProjectId