SQL joins cheat sheet

INNER, LEFT, FULL, CROSS and self-joins, with the row-count gotchas interviewers love.

INNER JOIN

LEFT JOIN

RIGHT JOIN

FULL JOIN

The core joins

INNER JOIN — only matching rows

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

LEFT JOIN — keep every left row

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

Unmatched right-side columns come back as NULL.

Anti-join — rows with no match

SELECT c.name
FROM customers c
LEFT JOIN orders o ON o.customer_id = c.id
WHERE o.id IS NULL;

Less common but asked

Self-join — compare rows in one table

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

CROSS JOIN — every combination

SELECT c.name, p.name
FROM customers c
CROSS JOIN products p;

Filter in ON vs WHERE

-- keeps all customers, only joins completed orders
SELECT c.name, o.id
FROM customers c
LEFT JOIN orders o
  ON o.customer_id = c.id AND o.status = 'completed';

Putting the status filter in WHERE silently turns the LEFT JOIN into an INNER JOIN.

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