Final answer:
A self-join is used in SQL when a table is joined with itself to compare rows within the same table, useful for hierarchical data or discovering duplicates.
Step-by-step explanation:
Sometimes you need to join a table to itself - this is known as a self-join. A self-join is a common technique used in SQL where a table is joined to itself to combine and compare rows within that same table. This is often necessary for comparing hierarchical data or finding duplicate rows within the same table.
For example, if you have a table of employees with a manager ID that references another employee, you might use a self-join to list each employee along with their manager's details. You would essentially be joining the employee table with itself based on the employee's manager ID matching another employee's ID.