FULL OUTER JOIN vs LEFT JOIN

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Accepted Answer

LEFT JOIN preserves all left-table rows and drops unmatched right-table rows. FULL OUTER JOIN goes further: it keeps unmatched rows from both sides, filling in NULLs wherever there is no match. A classic use case is reconciling two data sources - for example, finding customers in system A who are missing from system B and vice versa in one pass. Performance is not a distinguishing factor; FULL OUTER JOIN is simply the only join type that guarantees no row from either table is silently dropped.