Answer
A FULL OUTER JOIN keeps matched rows plus unmatched rows from both inputs. • Missing columns from the opposite side are filled with NULL. • It is useful when comparing two sets for matches and differences. • Not every database product supports this syntax directly.
💡 SQL Example
SELECT a.id AS old_id, b.id AS new_id FROM old_items a FULL OUTER JOIN new_items b ON b.id = a.id;
Result
Matched, removed, and added item identifiers
⚡ Quick Revision
FULL OUTER JOIN preserves unmatched rows from both sides.