SQL Joins, Explained Visually

The same two tables, four joins: see the SQL, the rows, and where the NULLs come from.

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query.sql

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

Only rows that match on BOTH sides. Dana (no orders) and order 103 (no customer) both vanish.

Shaded: the rows INNER JOIN keeps

Result (3 rows)

c.id	c.name	o.id	o.amount
1	Ava	100	40
1	Ava	101	25
2	Ben	102	60

customers

id	name
1	Ava
2	Ben
3	Cara
4	Dana

orders

id	customer_id	amount
100	1	40
101	1	25
102	2	60
103	5	10

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How it works

The one idea that makes joins click

A join is about which unmatched rows survive. INNER keeps only rows that match on both sides. LEFT keeps every left row and pads the missing right side with NULL. RIGHT does the mirror. FULL keeps everything from both. The data is identical; only the treatment of the rows that don't match changes.

Why NULLs appear (and the interview trap)

The NULLs in an outer join are the whole point: they mark rows with no match. The classic mistake is filtering on a right-table column in the WHERE clause of a LEFT JOIN, which silently turns it back into an INNER JOIN by dropping those NULL rows. Put such conditions in the ON clause instead.

A note on RIGHT and FULL

LEFT JOIN is far more common than RIGHT in real code. Any RIGHT JOIN can be rewritten as a LEFT by swapping the tables, which most people find easier to read. Some databases (older SQLite included) don't support RIGHT/FULL at all, so knowing the LEFT equivalent is genuinely useful.

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