Joins in sql

Joins are used to combine rows from two or more tables based on a related column between them. 

Types of joins

1. Inner Join

Purpose: Retrieves rows that have matching values in both tables.

Example:

SELECT * FROM customers 

INNER JOIN orders 

ON customers.id = orders.customer_id;


2. Left Join (Left Outer Join)

Purpose: Fetches all records from the left table and the matched records from the right table. If there’s no match, the result is NULL on the right side.

Example:

SELECT * FROM customers 

LEFT JOIN orders 

ON customers.id = orders.customer_id;


3. Right Join (Right Outer Join)

Purpose: Fetches all records from the right table and the matched records from the left table. If there’s no match, the result is NULL on the left side.

Example:

SELECT * FROM customers 

RIGHT JOIN orders 

ON customers.id = orders.customer_id;


4. Full Join (Full Outer Join)

Purpose: Returns matching rows from both tables, plus all remaining records from both tables—even if there is no match.

Example:

SELECT * FROM customers 

FULL OUTER JOIN orders 

ON customers.id = orders.customer_id;


5. Cross Join

Purpose: Combines each row from the first table with every row from the second table (producing a "cartesian product").

Example:

SELECT * FROM customers 

CROSS JOIN orders;



6. Exclusive Joins

Purpose: Returns only the records that do not have a match in the other table (exclusive of the overlap).

Left Exclusive Join:

SELECT * FROM customers 

LEFT JOIN orders 

ON customers.id = orders.customer_id 

WHERE orders.customer_id IS NULL;

Right Exclusive Join:

SELECT * FROM customers 

RIGHT JOIN orders 

ON customers.id = orders.customer_id 

WHERE customers.id IS NULL;


7. Self Join

Purpose: Joins a table to itself, useful for hierarchical data or when a table contains relationships within itself. Use table aliases to distinguish between the two instances.

Example:

SELECT s1.name AS mentor_name, s2.name AS student_name 

FROM student AS s1 

JOIN student AS s2 

ON s1.s_id = s2.mentor_id;

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