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SQL Fundamentals · SQL JOINs Explained: INNER vs LEFT JOIN and the Row Multiplication Trap

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Trick to keep in mind+3 XP

INNER JOIN keeps couples only: no partner, no row.

An inner join pairs rows from the two tables wherever the ON condition is true and keeps only those pairs. A row on either side with no match produces no pair, so it simply is not in the output. There is no error and no placeholder row. If you want to keep the unmatched side, you need an outer join.

Read it once, then a question tests whether the trick stuck.