MySQL中没有FULL OUTER JOIN。参见7.2.12. Outer Join Simplification和12.2.8.1. JOIN Syntax：
您可以使用UNION（从MySQL 4.0.0开始）模拟FULL OUTER JOIN：
其中两个表t1、t2：

SELECT * FROM t1
LEFT JOIN t2 ON t1.id = t2.id
UNION
SELECT * FROM t1
RIGHT JOIN t2 ON t1.id = t2.id

有三个表t1、t2、t3：

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

克莱图斯的回答不太对。UNION将删除FULL OUTER JOIN可能包含的重复记录。如果您需要使用类似以下内容的副本：

SELECT * FROM t1
LEFT JOIN t2 ON t1.id = t2.id
LEFT JOIN t3 ON t2.id = t3.id
LEFT JOIN t4 ON t3.id = t4.id
UNION ALL
SELECT * FROM t1
RIGHT JOIN t2 ON t1.id = t2.id
LEFT JOIN t3 ON t2.id = t3.id
LEFT JOIN t4 ON t3.id = t4.id
WHERE t1.id IS NULL
UNION ALL
SELECT * FROM t1
RIGHT JOIN t2 ON t1.id = t2.id
RIGHT JOIN t3 ON t2.id = t3.id
LEFT JOIN t4 ON t3.id = t4.id
WHERE t2.id IS NULL
UNION ALL
SELECT * FROM t1
RIGHT JOIN t2 ON t1.id = t2.id
RIGHT JOIN t3 ON t2.id = t3.id
RIGHT JOIN t4 ON t3.id = t4.id
WHERE t3.id IS NULL;

我刚刚为此做了一个小把戏：

(select 1 from DUAL) d
LEFT OUTER JOIN t1 ON t1.id = t2.id
LEFT OUTER JOIN t2 ON t1.id = t2.id

关键是，来自dual的查询是一个固定点，mysql可以将另外两个表外连接到该固定点

只需补充一下需要FULL OUTER JOIN三个表t1、t2、t3的情况。你可以依次做t1，t2，t3，左连接剩下的两个表，然后合并。

SELECT * FROM t1
LEFT JOIN t2 ON t1.id = t2.id
LEFT JOIN t3 ON t1.id = t3.id
UNION
SELECT * FROM t2
LEFT JOIN t1 ON t2.id = t1.id
LEFT JOIN t3 ON t2.id = t3.id
UNION
SELECT * FROM t3
LEFT JOIN t1 ON t3.id = t1.id
LEFT JOIN t2 ON t3.id = t2.id

只是为了补充这里提供的答案，我想分享这个连接4个表的例子，在所有SELECT语句上使用LEFT JOIN，还有一个更形象的例子。

SELECT doctor.name, professor.name, singer.name, actor.name
FROM doctor
LEFT JOIN professor ON professor.id = doctor.id
LEFT JOIN singer ON singer.id = doctor.id
LEFT JOIN actor ON actor.id = doctor.id
UNION
SELECT doctor.name, professor.name, singer.name, actor.name
FROM professor
LEFT JOIN doctor ON doctor.id = professor.id
LEFT JOIN singer ON singer.id = professor.id
LEFT JOIN actor ON actor.id = professor.id
UNION
SELECT doctor.name, professor.name, singer.name, actor.name
FROM singer
LEFT JOIN doctor ON doctor.id = singer.id
LEFT JOIN professor ON professor.id = singer.id
LEFT JOIN actor ON actor.id = singer.id
UNION
SELECT doctor.name, professor.name, singer.name, actor.name
FROM actor
LEFT JOIN doctor ON doctor.id = actor.id
LEFT JOIN professor ON professor.id = actor.id
LEFT JOIN singer ON singer.id = actor.id;