Self-Join

Techopedia explains Self-Join

As an example, there is a table named EMPLOYEES that contains three columns:

• employee name
• employee ID
• employee manager’s ID

Because the managers are also employees, the MANAGER_ID column also contains the ID of another employee that is also the manager. To write a query to extract the employee and manager names and IDs, the table must be logically split in half to run two separate queries: employees (first table) and managers (second table). This is achieved by running the following sample SQL query:

SELECT a.employee_name, b.employee_name as Manager_name
FROM employees as a, employees as b
WHERE a.manager_id = b.employee_id

Understanding the self-join concept and circumstances is essential to grasping the above SQL statement.

In the example, the second EMPLOYEES table is given the alias b, which actually is a subset of the full EMPLOYEES table. However, the WHERE condition forces the first EMPLOYEES table to query the employee manager in the second EMPLOYEES table.