The CData Power BI Connector for Slack supports standard SQL joins like the following examples.
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
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