A self-join is a type of join in which a table is joined to itself. To perform a self-join, you need to specify the table name twice in the FROM clause, and give each instance of the table a different alias. You can then join the two instances of the table using a JOIN clause, and use a WHERE clause to specify the relationship between the rows. Self-joins are the go-to technique for any data analysis that involves pairs of the same thing, like identifying pairs of products that are frequently purchased together like in this Walmart SQL interview question. For another example, say you were doing an HR analytics project and needed to analyze how much all Uber employees in the same department interact with each other. Here's a self-join query you could use to retrieve all pairs of Uber employees who work in the same department: SELECT e1.name AS employee1, e2.name AS employee2 FROM uber_employees AS e1 JOIN uber_employees AS e2 ON e1.department_id = e2.department_id WHERE e1.id < e2.id; This query returns all pairs of Uber employees who work in the same department, and excludes pairs where the employee's id is the same (since this would represent the same Uber employee being paired with themselves). ---