How Can I Emulate a FULL JOIN in MySQL Using LEFT and RIGHT JOINs?

Emulating MySQL FULL JOIN with LEFT and RIGHT JOINS

MySQL does not natively support FULL JOIN operations. However, it is possible to achieve similar results using a combination of LEFT JOIN and RIGHT JOIN.

Understanding the Problem

The goal is to return a result set that includes all records from both tables, Persons and Orders, even if the matching fields (P_Id) are null in either table.

Solution

To simulate a FULL JOIN, use the following query:

SELECT  p.LastName, p.FirstName, o.OrderNo
FROM    Persons p
LEFT JOIN
        Orders o
ON      p.P_Id = o.P_Id
UNION ALL
SELECT  p.LastName, p.FirstName, o.OrderNo
FROM    Persons p
RIGHT JOIN
        Orders o
ON      p.P_Id = o.P_Id
WHERE   p.P_Id IS NULL
ORDER BY
        p.LastName;

Explanation

• The LEFT JOIN returns all rows from the Persons table, including those with matching P_Id values in the Orders table and those without.
• The UNION ALL operation combines the results of the LEFT JOIN with the results of the RIGHT JOIN, which returns all rows from the Orders table, including those with matching P_Id values in the Persons table and those without.
• The WHERE clause filters out any rows from the Persons table that have a null P_Id, ensuring that all rows from the LEFT JOIN are included.
• The ORDER BY clause sorts the results by the LastName column.

This query produces a result set identical to the one described in the problem statement:

LastName	FirstName	OrderNo
Hansen	Ola	22456
Hansen	Ola	24562
Pettersen	Kari	77895
Pettersen	Kari	44678
Svendsen	Tove	34764

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