Simulate FULL OUTER JOIN in MySQL
MySQL itself does not support FULL OUTER JOIN. However, you can simulate it using a combination of LEFT JOIN, RIGHT JOIN, and set operators.
LIMITED FULL OUTER JOIN
For cases where FULL OUTER JOIN does not produce duplicate rows, you can use:
SELECT * FROM t1 LEFT JOIN t2 ON t1.id = t2.id UNION SELECT * FROM t1 RIGHT JOIN t2 ON t1.id = t2.id
The UNION operator eliminates duplicate rows introduced by this query pattern.
General FULL OUTER JOIN
For the common case where duplicate rows may occur, use:
SELECT * FROM t1 LEFT JOIN t2 ON t1.id = t2.id UNION ALL SELECT * FROM t1 RIGHT JOIN t2 ON t1.id = t2.id WHERE t1.id IS NULL
The UNION ALL operator combines the results of two JOINs, and the additional WHERE clause ensures that only rows with t1.id NULL (not matched in the LEFT JOIN) are added to the final result, thus eliminating duplicates.
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