Use:

SELECT * FROM t1 FULL OUTER JOIN t2 ON t1.id = t2.id;

可以按如下方式重新创建：

 SELECT t1.*, t2.*
 FROM (SELECT * FROM t1 UNION SELECT name FROM t2) tmp
 LEFT JOIN t1 ON t1.id = tmp.id
 LEFT JOIN t2 ON t2.id = tmp.id;

使用UNION或UNION ALL答案不能涵盖基表具有重复条目的边缘情况。

说明：

UNION或UNION ALL无法覆盖边缘情况。我们无法在MySQL上测试这一点，因为它不支持完整的外部连接，但我们可以在一个支持它的数据库上说明这一点：

 WITH cte_t1 AS
 (
     SELECT 1 AS id1
     UNION ALL SELECT 2
     UNION ALL SELECT 5
     UNION ALL SELECT 6
     UNION ALL SELECT 6
 ),
cte_t2 AS
(
     SELECT 3 AS id2
     UNION ALL SELECT 4
     UNION ALL SELECT 5
     UNION ALL SELECT 6
     UNION ALL SELECT 6
)
SELECT  *  FROM  cte_t1 t1 FULL OUTER JOIN cte_t2 t2 ON t1.id1 = t2.id2;

这给了我们这样的答案：

id1  id2
1  NULL
2  NULL
NULL  3
NULL  4
5  5
6  6
6  6
6  6
6  6

UNION解决方案：

SELECT  * FROM  cte_t1 t1 LEFT OUTER JOIN cte_t2 t2 ON t1.id1 = t2.id2
UNION    
SELECT  * FROM cte_t1 t1 RIGHT OUTER JOIN cte_t2 t2 ON t1.id1 = t2.id2

给出错误答案：

 id1  id2
NULL  3
NULL  4
1  NULL
2  NULL
5  5
6  6

UNION ALL解决方案：

SELECT  * FROM cte_t1 t1 LEFT OUTER join cte_t2 t2 ON t1.id1 = t2.id2
UNION ALL
SELECT  * FROM  cte_t1 t1 RIGHT OUTER JOIN cte_t2 t2 ON t1.id1 = t2.id2

也不正确。

id1  id2
1  NULL
2  NULL
5  5
6  6
6  6
6  6
6  6
NULL  3
NULL  4
5  5
6  6
6  6
6  6
6  6

鉴于此查询：

SELECT t1.*, t2.*
FROM (SELECT * FROM t1 UNION SELECT name FROM t2) tmp
LEFT JOIN t1 ON t1.id = tmp.id
LEFT JOIN t2 ON t2.id = tmp.id;

给出以下内容：

id1  id2
1  NULL
2  NULL
NULL  3
NULL  4
5  5
6  6
6  6
6  6
6  6

顺序不同，但在其他方面与正确答案匹配。

前面的答案实际上都不正确，因为当存在重复值时，它们不遵循语义。

对于查询，例如（来自此副本）：

SELECT * FROM t1 FULL OUTER JOIN t2 ON t1.Name = t2.Name;

正确的等效值为：

SELECT t1.*, t2.*
FROM (SELECT name FROM t1 UNION  -- This is intentionally UNION to remove duplicates
      SELECT name FROM t2
     ) n LEFT JOIN
     t1
     ON t1.name = n.name LEFT JOIN
     t2
     ON t2.name = n.name;

如果需要使用NULL值（这可能也是必要的），则使用NULL安全比较运算符<=>而不是=。

Use:

SELECT * FROM t1 FULL OUTER JOIN t2 ON t1.id = t2.id;

可以按如下方式重新创建：

 SELECT t1.*, t2.*
 FROM (SELECT * FROM t1 UNION SELECT name FROM t2) tmp
 LEFT JOIN t1 ON t1.id = tmp.id
 LEFT JOIN t2 ON t2.id = tmp.id;

使用UNION或UNION ALL答案不能涵盖基表具有重复条目的边缘情况。

说明：

UNION或UNION ALL无法覆盖边缘情况。我们无法在MySQL上测试这一点，因为它不支持完整的外部连接，但我们可以在一个支持它的数据库上说明这一点：

 WITH cte_t1 AS
 (
     SELECT 1 AS id1
     UNION ALL SELECT 2
     UNION ALL SELECT 5
     UNION ALL SELECT 6
     UNION ALL SELECT 6
 ),
cte_t2 AS
(
     SELECT 3 AS id2
     UNION ALL SELECT 4
     UNION ALL SELECT 5
     UNION ALL SELECT 6
     UNION ALL SELECT 6
)
SELECT  *  FROM  cte_t1 t1 FULL OUTER JOIN cte_t2 t2 ON t1.id1 = t2.id2;

这给了我们这样的答案：

id1  id2
1  NULL
2  NULL
NULL  3
NULL  4
5  5
6  6
6  6
6  6
6  6

UNION解决方案：

SELECT  * FROM  cte_t1 t1 LEFT OUTER JOIN cte_t2 t2 ON t1.id1 = t2.id2
UNION    
SELECT  * FROM cte_t1 t1 RIGHT OUTER JOIN cte_t2 t2 ON t1.id1 = t2.id2

给出错误答案：

 id1  id2
NULL  3
NULL  4
1  NULL
2  NULL
5  5
6  6

UNION ALL解决方案：

SELECT  * FROM cte_t1 t1 LEFT OUTER join cte_t2 t2 ON t1.id1 = t2.id2
UNION ALL
SELECT  * FROM  cte_t1 t1 RIGHT OUTER JOIN cte_t2 t2 ON t1.id1 = t2.id2

也不正确。

id1  id2
1  NULL
2  NULL
5  5
6  6
6  6
6  6
6  6
NULL  3
NULL  4
5  5
6  6
6  6
6  6
6  6

鉴于此查询：

SELECT t1.*, t2.*
FROM (SELECT * FROM t1 UNION SELECT name FROM t2) tmp
LEFT JOIN t1 ON t1.id = tmp.id
LEFT JOIN t2 ON t2.id = tmp.id;

给出以下内容：

id1  id2
1  NULL
2  NULL
NULL  3
NULL  4
5  5
6  6
6  6
6  6
6  6

顺序不同，但在其他方面与正确答案匹配。

使用交叉联接解决方案：

SELECT t1.*, t2.*
FROM table1 t1
INNER JOIN table2 t2 
ON 1=1;

我修复了响应，工作包括所有行（基于Pavle Lekic的响应）：

    (
    SELECT a.* FROM tablea a
    LEFT JOIN tableb b ON a.`key` = b.key
    WHERE b.`key` is null
    )
    UNION ALL
    (
    SELECT a.* FROM tablea a
    LEFT JOIN tableb b ON a.`key` = b.key
    where  a.`key` = b.`key`
    )
    UNION ALL
    (
    SELECT b.* FROM tablea a
    right JOIN tableb b ON b.`key` = a.key
    WHERE a.`key` is null
    );

您可以只转换一个完整的外部联接，例如。

SELECT fields
FROM firsttable
FULL OUTER JOIN secondtable ON joincondition

进入：

SELECT fields
FROM firsttable
LEFT JOIN secondtable ON joincondition
UNION ALL
SELECT fields (replacing any fields from firsttable with NULL)
FROM secondtable
WHERE NOT EXISTS (SELECT 1 FROM firsttable WHERE joincondition)

或者，如果firsttable中至少有一列（比如foo）不为NULL，则可以执行以下操作：

SELECT fields
FROM firsttable
LEFT JOIN secondtable ON joincondition
UNION ALL
SELECT fields
FROM firsttable
RIGHT JOIN secondtable ON joincondition
WHERE firsttable.foo IS NULL