Self joins for hierarchical data

A self join is when a table is joined to itself. This sounds strange at first, but it's essential for querying hierarchical data, comparing rows within the same table, and finding related records. Why Self Join? Self joins solve problems where relationships exist within a single table. The most common example: an employee_metrics table where each employee has a manager_id that references another row in the same table. Alice is the CEO (no manager). Bob and Carol report to Alice. Dave and Eve report to Bob. Frank reports to Carol. To show each employee with their manager's name, we join the table to itself. Self Join Syntax Use table aliases to distinguish the two "copies" of the table. Think of it as creating two separate reference points into the same data: Watch how the table is joined t