Why Does My MySQL Query Fail with a Syntax Error on FULL OUTER JOIN?

MySQL's Lack of FULL OUTER JOIN and the Workaround

Problem:

A MySQL query using FULL OUTER JOIN resulted in a syntax error. The query attempted to join multiple tables using FULL OUTER JOIN, a syntax seemingly correct but unsupported by MySQL.

Solution:

MySQL doesn't directly support FULL OUTER JOIN. The solution involves emulating this functionality using a combination of LEFT JOIN and RIGHT JOIN with UNION statements.

Emulating FULL OUTER JOIN:

For two tables (t1 and t2), a FULL OUTER JOIN is replicated like this:

<code class="language-sql">SELECT * FROM t1
LEFT JOIN t2 ON t1.id = t2.id
UNION
SELECT * FROM t1
RIGHT JOIN t2 ON t1.id = t2.id;</code>

This approach extends to multiple tables. For three tables (t1, t2, t3), the equivalent would be:

<code class="language-sql">SELECT * FROM t1
LEFT JOIN t2 ON t1.id = t2.id
LEFT JOIN t3 ON t2.id = t3.id
UNION
SELECT * FROM t1
RIGHT JOIN t2 ON t1.id = t2.id
LEFT JOIN t3 ON t2.id = t3.id
UNION
SELECT * FROM t1
RIGHT JOIN t2 ON t1.id = t2.id
RIGHT JOIN t3 ON t2.id = t3.id;</code>

This recursive application of LEFT JOIN, RIGHT JOIN, and UNION effectively mimics the behavior of a FULL OUTER JOIN in MySQL, providing a practical workaround for this limitation.

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