🔗 SQL CROSS JOIN and SELF JOIN Explained with Examples

🔄 Lab 2.4: SQL CROSS JOIN and SELF JOIN

This lab focuses on two special types of joins in SQL — CROSS JOIN and SELF JOIN. We’ll use Bangladeshi employee data to understand how each works.

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📁 Sample Employees Table

emp_id	emp_name	manager_id
1	Rakib	3
2	Tamanna	3
3	Imran (Manager)	NULL

🔁 1. CROSS JOIN

SELECT a.emp_name AS emp1, b.emp_name AS emp2
FROM employees a
CROSS JOIN employees b;

This joins every row from the first table with every row from the second table. Useful for combinations or testing all pairs.

• If 3 employees exist, you get 3 × 3 = 9 rows
• This is also called a **Cartesian Product**

🎯 Example Use Case:

Generate all possible pairs of employees for assigning as team leads.

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🔁 2. SELF JOIN

SELECT e.emp_name AS Employee, m.emp_name AS Manager
FROM employees e
LEFT JOIN employees m
ON e.manager_id = m.emp_id;

A SELF JOIN joins a table with itself. Here, we’re joining the employee table to itself to match each employee with their manager.

🎯 Output:

• Rakib → Manager: Imran
• Tamanna → Manager: Imran
• Imran → Manager: NULL

🎯 Example Use Case:

Show who reports to whom in an organizational hierarchy.

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📌 Summary

• CROSS JOIN — All combinations between two tables
• SELF JOIN — Join table with itself to show relationships like manager-employee

✅ These joins are less common but very powerful in data modeling and reporting. Practice them using real datasets like employee-manager relationships.

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