The CData Power BI Connector for Slack 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 Channels.Name, Users.FullName FROM Channels, Users WHERE Channels.Creator=Users.Id

Left Join

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

SELECT Channels.Name, Users.FullName FROM Channels LEFT OUTER JOIN Users ON Channels.Creator=Users.Id