Efficient way to simulate full outer join in MySQL?

Question

According to Google search: since MySQL does not support full outer join, it could be simulated via union and/or union all. But both of these either remove genuine duplicate

Answer 1

You can use a LEFT JOIN and a RIGHT JOIN:

SELECT * FROM tableA LEFT JOIN tableB ON tableA.b_id = tableB.id
UNION ALL
SELECT * FROM tableA RIGHT JOIN tableB ON tableA.b_id = tableB.id
WHERE tableA.b_id IS NULL

There is also some information on Wikipedia about this topic: Full outer join.

The Wikipedia article suggests using a UNION in MySQL. This is slightly slower than UNION ALL, but more importantly it won't always give the correct result - it will remove duplicated rows from the output. So prefer to use UNION ALL instead of UNION here.