But what if you want to perform a JOIN operation on a single column? In this case, you can use a self-join.

A self-join is a JOIN operation where a table is joined with itself. By aliasing the table with different names, you can treat it as if it were two separate tables.

Let’s consider an example to illustrate this concept. Imagine you have a table called “employees” with the following columns: “employee_id”, “employee_name”, and “manager_id”. The “manager_id” column stores the ID of the employee’s manager.

To get the employee’s name along with the name of their manager, you can use a self-join on the “employees” table. Here’s an example query in SQL:

SELECT e.employee_name AS employee, m.employee_name AS manager
FROM employees e
JOIN employees m ON e.manager_id = m.employee_id

In the query above, we aliased the table “employees” as “e” for the employee’s information and as “m” for the manager’s information. We then joined the table on the “manager_id” column of the “e” alias, matching it with the “employee_id” column of the “m” alias.

By executing this query, you will get the list of employees along with their corresponding manager’s name.

Self-joins can be handy when you need to relate records within the same table. It allows you to retrieve hierarchical data or compare values across different rows.

Next time you come across a scenario where you need to perform a JOIN on a single column, consider using a self-join to accomplish the task efficiently.

#database #SQL