A self join is basically when a table is joined to itself. The way you should visualize a self join for a given table is by imagining that a join is performed between two identical copies of that table. And that is exactly why it is called a self join – because of the fact that it’s just the same table being joined to anothercopy of itself rather than being joined with a different table.
FROM employee e1, employee e2
WHERE e1.Location = e2.Location