SQL Self Join

A Self Join in SQL is a type of join where a table is joined with itself. It is useful for comparing rows within the same table, especially in cases involving hierarchical or relational data- such as retrieving employee-manager relationships from an employee table.

Syntax:

SELECT columns
FROM table AS alias1
JOIN table AS alias2 ON alias1.column = alias2.column;

Explanation:

• SELECT columns: With the help of this we specify the columns you want to retrieve from the self-joined table.
• FROM table AS alias1: With the help of this we specify the name of the table you want to join with itself.
• JOIN table AS alias2: In this, we use the JOIN keyword to show that we are performing a self-join on the same table.

Example: SQL Self Join to Retrieve Employees and Their Managers

Assume that we have a table called "GFGemployees" with the columns employee_id, employee_name, and manager_id. Each employee in the company is assigned a manager, and using the manager-ids, we can identify each employee. We need to extract the list of employees along with the names of their managers because the manager_id column contains the manager ID for each employee

Step 1: First, we need to create the "GFGemployees" table with following query in SQL.

CREATE TABLE GFGemployees(employee_id
INT PRIMARY KEY, employee_name VARCHAR(50), manager_id INT);

Step 2: Now we will add data into the 'GFGemployees' table using INSERT INTO statement:

INSERT INTO GFGemployees (employee_id, employee_name, manager_id)
VALUES (1, 'Zaid', 3), (2, 'Rahul', 3), (3, 'Raman', 4),
(4, 'Kamran', NULL), (5, 'Farhan', 4);

Output:

Step 3: Explanation and implementation of Self Join

Now, we need to perform self join on the table we created i.e."GFGemployees" in order to retrieve the list of employees and their corresponding managers name and for that we need to write a query, where we will create two different aliases for the "GFGemployees" table as "e" which will represent the GFG employee's information and "m" will represent the manager's information. This way by joining the table with itself using the manager_id and employee_id columns, we can generate relationship between employees and their managers.

Step 4: Query for Self-join

SELECT e.employee_name AS employee,
m.employee_name AS managerFROM
GFGemployees AS e JOIN GFGemployees
AS m ON e.manager_id = m.employee_id;

Output:

The resultant table after performing self join will be as follows:

Applications of SQL Self Join

• Hierarchical Data: Self joins are particularly useful when working with hierarchical data such as organizational structures, where each employee has a manager.
• Finding Relationships: Self joins can be used to find relationships within the same table, such as identifying employees with similar attributes or roles.
• Data Comparison: Self joins allow comparing records within the same table based on specific conditions, like comparing sales figures of employees working in the same department.