Which join returns every row from both tables, filling the columns of the other table with NULL wherever no match exists?
Correct answer A. FULL OUTER JOIN
Explanation
A FULL OUTER JOIN returns matched pairs plus unmatched rows from the left table and unmatched rows from the right table, padding missing values with NULL. A LEFT OUTER JOIN keeps unmatched rows from the left side only. An INNER JOIN keeps matched pairs only. A CROSS JOIN pairs every row with every row and has no matching condition at all. Note that MySQL lacks FULL OUTER JOIN and emulates it with UNION.
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