The CData JDBC Driver 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