A self join is a type of join operation in SQL where a table is joined to itself. This is different from joining two separate tables. Instead, you're essentially creating a temporary virtual copy of the table to compare rows within the same table. This is particularly useful when you need to analyze relationships within a single table, such as finding hierarchical structures, identifying pairs of related items, or comparing data across different time periods within the same table. For example, in an employee database, you might want to find all employees who report to the same manager. Or, in a schedule database, you might want to find overlapping appointments. Self joins are powerful tools for analyzing data within a single table.