When the full outer join is used?
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You want all unmatched data from both tables.
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One of the tables has more data than the other.
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You want all matched data from both tables.
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You want all matched and unmatched data from only one table.
FULL OUTER JOIN returns all matched and unmatched records from both tables. Unlike INNER JOIN (only matched) or LEFT/RIGHT OUTER JOIN (all from one side + matched from other), FULL OUTER JOIN ensures no data is lost from either table. Rows without matches show NULL values for the non-matching side's columns.
A FULL OUTER JOIN returns all matched rows (like an inner join) plus all unmatched rows from both the left and right tables, with NULLs filling the missing side — it is effectively a LEFT JOIN and RIGHT JOIN combined. Technically the most complete description would be "all matched AND unmatched data from both tables," but that exact phrasing is not offered among the four options. Of the choices given, "You want all unmatched data from both tables" captures the join's defining, distinguishing use case (retrieving unmatched rows from both sides simultaneously, which no other join type does) and is the standard simplified phrasing used in many SQL quiz banks and tutorials describing why you'd reach for a full outer join over an inner/left/right join. The other options describe an inner join ("matched only"), an irrelevant condition (table size), or a mismatched single-table scope.