完全外部加入

FULL OUTER JOIN 在两个表之间执行连接，这两个表需要显式连接子句，但不排除任一表中不匹配的行。换句话说，它返回每个表中的所有行。

例：

SELECT                                     
      *                                    
FROM                                       
      EMPLOYEE FULL OUTER JOIN DEPT 
      ON EMPLOYEE.DEPTNO = DEPT.DEPTNO;         

以下是两个表之间的完全外部联接的示例：

样本表： 员工

+-----------+---------+
|    NAME   |  DEPTNO |
+-----------+---------+
|     A     |    2    |
|     B     |    1    |
|     C     |    3    |
|     D     |    2    |
|     E     |    1    |
|     F     |    1    |
|     G     |    4    |
|     H     |    4    |
+-----------+---------+

样本表： DEPT

+---------+--------------+
|  DEPTNO |   DEPTNAME   |
+---------+--------------+
|    1    |  ACCOUNTING  |
|    2    |    FINANCE   |
|    5    |   MARKETING  |
|    6    |      HR      |
+---------+--------------+

现在，如果执行查询：

SELECT
      *
FROM
      EMPLOYEE FULL OUTER JOIN DEPT
      ON EMPLOYEE.DEPTNO = DEPT.DEPTNO;

输出

+-----------+---------+---------+--------------+
|    NAME   |  DEPTNO |  DEPTNO |   DEPTNAME   |
+-----------+---------+---------+--------------+
|     A     |    2    |    2    |    FINANCE   |
|     B     |    1    |    1    |  ACCOUNTING  |
|     C     |    3    |         |              |
|     D     |    2    |    2    |    FINANCE   |
|     E     |    1    |    1    |  ACCOUNTING  |
|     F     |    1    |    1    |  ACCOUNTING  |
|     G     |    4    |         |              |
|     H     |    4    |         |              |
|           |         |    6    |      HR      |
|           |         |    5    |   MARKETING  |
+-----------+---------+---------+--------------+

这里不匹配的列保持为 NULL。