我注意到Oracle 11中FULL OUTER JOIN的奇怪行为。我从HR模式（尤其是EMPLOYEES和DEPARTMENTS）联接表。

例如，以下查询返回123行：

    SELECT * FROM employees e
    FULL JOIN departments d ON e.department_id = d.department_id

但是，难以理解的是-当我在select子句中放置一组特定的列时，查询将返回122行（缺少行的是缺少部门的雇员的行-左连接另外返回的行）与内部联接相比）：

    SELECT first_name, last_name, department_name FROM employees e
    FULL JOIN departments d on e.department_id = d.department_id

即使我计算行数也返回122（COUNT(*)）！！！到底是怎么回事？SELECT *和之间有什么区别SELECT COUNT(*)？

的解释计划SELECT * ...：

SELECT STATEMENT                                      122
  VIEW                 VW_FOJ_0                       122
    HASH JOIN                          FULL OUTER     122
      Access Predicates
        E.DEPARTMENT_ID = D.DEPARTMENT_ID
      TABLE ACCESS     DEPARTMENTS     FULL            27
      TABLE ACCESS     EMPLOYEES       FULL           107

和为SELECT COUNT(*) ...：

SELECT STATEMENT                                             1
  SORT                                     AGGREGATE         1
    VIEW               VW_FOJ_0                            122
      HASH JOIN                            FULL OUTER      122
        Access Predicates
          E.DEPARTMENT_ID = D.DEPARTMENT_ID
        INDEX          DEPT_ID_PK          FAST FULL SCAN   27
        INDEX          EMP_DEPARTMENT_IX   FAST FULL SCAN  107