A self-join is often very useful to convert a hierarchical structure to a flat structure. It is used to join a table to itself as like if that is the second table.
In SQL, a self-join is a regular join, but the table is joined with itself. This can be useful when you want to combine rows from the same table based on a related column. The requirement for a self-join is that there must be a column in the table that establishes a relationship between the rows within the same table.
For example, consider a table called “employees” with columns like “employee_id” and “manager_id.” In this case, you might use a self-join to retrieve information about employees and their managers by matching the “employee_id” with the “manager_id” in the same table.
Here’s an example query for a self-join:
SELECT e.employee_id, e.employee_name, m.employee_name AS manager_name
FROM employees e
JOIN employees m ON e.manager_id = m.employee_id;
In this query, we are joining the “employees” table with itself based on the relationship between “employee_id” and “manager_id.” The result includes columns for employee information and the name of their respective managers.
The requirement for a self-join is having a column that establishes a relationship within the same table, typically through foreign key relationships or some form of hierarchical structure.