The CData Excel Add-In for GitHub 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 Commits.AuthorName, CommitComments.Body FROM Commits, CommitComments WHERE Commits.Sha=CommitComments.CommitSha

Left Join

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

SELECT Commits.AuthorName, CommitComments.Body FROM Commits LEFT OUTER JOIN CommitComments ON Commits.Sha=CommitComments.CommitSha