SQL JOIN Helper

Visual reference for INNER / LEFT / RIGHT / FULL / CROSS / SELF joins with live-simulated results.

LEFT (OUTER) JOIN

All rows from the left; right-side columns are NULL when there's no match.

Example query

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

users

id	name
1	Alice
2	Bob
3	Carol

orders

id	user_id	total
100	1	42
101	1	7
102	2	15
103	9	99

Result (4 rows)

u.id	u.name	o.id	o.total
1	Alice	100	42
1	Alice	101	7
2	Bob	102	15
3	Carol	NULL	NULL

Mental model

Every JOIN starts as a Cartesian product (every left row × every right row) and is then filtered by the ON condition. The flavor (INNER, LEFT, etc.) controls what happens to rows that don't match.

Pick by question

“Only matching rows” → INNER
“All on the left plus matches” → LEFT
“All on the right plus matches” → RIGHT
“Everything from both sides” → FULL OUTER
“Every pairing” → CROSS

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Mental model

Pick by question

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