In SQL, a self join is used to join a table with itself. This can be useful when you want to compare records in the same table, such as when comparing employee information to find managers or when comparing customer information to find referrals.

How to Use Self Join in SQL

To perform a self join, you need to use an alias for the table to distinguish between the two instances of the same table. Here’s a basic example of a self join in SQL:

SELECT e1.employee_name AS employee, e2.employee_name AS manager
FROM employees e1
JOIN employees e2 ON e1.manager_id = e2.employee_id;

In this example, e1 and e2 are the aliases for the employees table. The join condition e1.manager_id = e2.employee_id compares the manager_id in the first instance of the employees table with the employee_id in the second instance to find the corresponding manager for each employee.

Benefits of Self Join

Using self join in SQL can help simplify complex queries and provide insight into hierarchical relationships within the same table.

Conclusion

Self join in SQL allows you to compare records within the same table, making it a useful tool for analyzing hierarchical relationships and other scenarios that involve comparing data within a single table.

For more information, you can refer to the SQL Self Join documentation.