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Data & Analytics · Intermediate

FULL OUTER JOIN

A join that returns all rows from both tables, matching where possible and filling NULLs on whichever side has no match.

Definition

FULL OUTER JOIN combines the results of a left and a right join. It is useful for reconciling two sources to find records that exist only on one side. MySQL does not support it natively; a UNION of LEFT and RIGHT joins emulates it.

Example

SELECT * FROM SystemA a FULL OUTER JOIN SystemB b ON a.ID = b.ID WHERE a.ID IS NULL OR b.ID IS NULL;

Exam tip

For 'show records missing from either system', use FULL OUTER JOIN with IS NULL filters.

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