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我注意到Oracle 11中出现了一个奇怪的FULL OUTER JOIN行为.我正在从HR模式加入表,特别是EMPLOYEES和DEPARTMENTS.

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,要理解的是什么 - 当我在select子句中放入一组特定的列时,查询将返回122行(缺少的行是针对没有分配部门的员工 - 另一个是使用左连接返回的行)与内连接相比):

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

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

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

并为SELECT …