Final answer:
A self join is a join operation where a table in a database is joined to itself to compare values within the same table. It is often used to find rows that share a common characteristic or to display hierarchical relationships, such as employees and their managers.
Step-by-step explanation:
When a table in a database is linked to itself, the type of join that is performed is called a self join. This is a special case of a relational database join operation that is used to compare values within the same table. A self join can be used to find rows in a table that share a common characteristic, such as finding pairs of employees who work in the same department within an employee table.
Examples of self join: Consider a scenario where you have an employee table with the following columns: employee_ id, name, and manager_ id, where manager_ id refers to the employee_ id of the employee's manager. To list employees along with their managers, you would perform a self join like so: Select all fields from the employee table and give it an alias 'e1'. Join the employee table to itself, aliasing the table again as 'e2'. On the join condition, specify that e1.manager_id equals e2.employee_id. This will return a result set that shows each employee along with their respective manager.