How to Emulate a FULL JOIN in MySQL?

Understanding the Limitations of MySQL's FULL JOIN

In MySQL, unlike some other relational databases, there is no direct support for the FULL JOIN operation. This means that achieving the desired result stated in the question requires a different approach.

Emulating a FULL JOIN in MySQL

To emulate a FULL JOIN in MySQL, the recommended workaround involves a combination of LEFT JOIN and RIGHT JOIN. Here's how it's done:

SELECT
  t_13.value AS val13,
  t_17.value AS val17
FROM
  t_13
LEFT JOIN
  t_17
ON
  t_13.value = t_17.value
UNION ALL
SELECT
  t_13.value AS val13,
  t_17.value AS val17
FROM
  t_13
RIGHT JOIN
  t_17
ON
  t_13.value = t_17.value
WHERE
  t_13.value IS NULL
ORDER BY
  COALESCE(val13, val17)
LIMIT 30;

This query effectively produces the Cartesian product of the two tables (t_13 and t_17), and then filters out the rows where both values are NULL. The result is a table that includes all rows from both tables, even those without matching values in the other table, ensuring that all possible combinations are represented.

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