What is a JOIN? Explain types.

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In SQL, a JOIN is used to combine data from two or more tables based on a related column (usually a primary key–foreign key relationship). Joins help retrieve meaningful information spread across multiple tables.

🔑 Types of Joins in SQL

1. INNER JOIN

   • Returns only the rows that have matching values in both tables.

   • Example:

     SELECT employees.name, departments.dept_name
     FROM employees
     INNER JOIN departments
     ON employees.dept_id = departments.id;
     

   • ✅ Output: Only employees who belong to an existing department.

2. LEFT JOIN (LEFT OUTER JOIN)

   • Returns all rows from the left table and matching rows from the right.

   • If no match, NULLs are returned for right table columns.

   • Example: Show all employees, even those without a department.

3. RIGHT JOIN (RIGHT OUTER JOIN)

   • Returns all rows from the right table and matching rows from the left.

   • If no match, NULLs are returned for left table columns.

   • Example: Show all departments, even if no employees exist in them.

4. FULL JOIN (FULL OUTER JOIN)

   • Returns all rows when there is a match in either left or right table.

   • Non-matching rows from both sides are filled with NULLs.

   • Example: List all employees and departments, even if they don’t match.

5. CROSS JOIN

   • Produces the Cartesian product of both tables (every row of A with every row of B).

   • Example: If table A has 3 rows and table B has 4, result = 12 rows.

6. SELF JOIN

   • A table joined with itself.

   • Useful for hierarchical or recursive data (e.g., employees with their managers).

⚡ Summary Table

JOIN Type	Description	Returns
INNER JOIN	Matching rows only	Intersection
LEFT JOIN	All rows from left + matches	Left + matches
RIGHT JOIN	All rows from right + matches	Right + matches
FULL JOIN	All rows from both	Union
CROSS JOIN	Cartesian product	All combinations
SELF JOIN	Table with itself	Hierarchical data

👉 In short: JOINs help combine relational data. Choose INNER for exact matches, LEFT/RIGHT/FULL for outer coverage, CROSS for all combinations, and SELF for hierarchical relationships.

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