About SQL Join Maze

SQL Join Maze shows you two or more small data tables plus a target result set and asks you to pick the SQL join statement that produces exactly that output. Topics span INNER JOIN, LEFT JOIN, RIGHT JOIN, FULL OUTER JOIN, anti-join patterns (LEFT JOIN … WHERE r.id IS NULL), semi-joins, self-joins, and tricky ON vs WHERE placement that flips outer to inner behavior.

Why it helps

• Build intuition for NULL semantics — when a row disappears vs when it stays with NULLs.
• Understand cardinality blow-up from non-unique join keys before it hits production queries.
• Learn the difference between filtering in the ON clause vs the WHERE clause for outer joins.
• Use the related SQL Join Builder to iterate on the correct query with your own data.

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