LEFT & RIGHT JOIN

LEFT JOIN

LEFT JOIN keeps every row from the left table, filling unmatched right-side columns with NULL.

SELECT c.name, o.id AS order_id
FROM customers c
LEFT JOIN orders o ON o.customer_id = c.id;

Output:

name        | order_id
Asha Rao    | 1
Rahul Mehta | 2
Priya Nair  | NULL

RIGHT JOIN

RIGHT JOIN is the mirror of LEFT JOIN — it keeps every row from the right table instead. Most people just swap table order and use LEFT JOIN.

Finding unmatched rows

Add WHERE o.id IS NULL after a LEFT JOIN to find customers with no orders at all.