JOIN Queries

Code

-- INNER JOIN
SELECT u.name, o.order_date, o.total
FROM users u
INNER JOIN orders o ON u.id = o.user_id;

-- LEFT JOIN
SELECT u.name, o.order_date
FROM users u
LEFT JOIN orders o ON u.id = o.user_id;

-- Multi-table JOIN
SELECT u.name, p.title, oi.quantity
FROM users u
JOIN orders o ON u.id = o.user_id
JOIN order_items oi ON o.id = oi.order_id
JOIN products p ON oi.product_id = p.id;

-- Self join
SELECT e.name AS employee, m.name AS manager
FROM employees e
LEFT JOIN employees m ON e.manager_id = m.id;

Explanation

JOIN combines data from multiple tables; INNER JOIN returns only matching rows, LEFT JOIN keeps all left table rows.

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