oracle中左关联,右关联区别

Oracle中可以使用“(+) ”来表示，9i可以使用LEFT/RIGHT/FULL OUTER JOIN

LEFT OUTER JOIN：左外关联

    SELECT e.last_name, e.department_id, d.department_name   
    FROM employees e   
    LEFT OUTER JOIN departments d   
    ON (e.department_id = d.department_id);   
    等价于   
    SELECT e.last_name, e.department_id, d.department_name   
    FROM employees e, departments d   
    WHERE e.department_id=d.department_id(+)   

结果为：所有员工及对应部门的记录，包括没有对应部门编号department_id的员工记录。

RIGHT OUTER JOIN：右外关联

    SELECT e.last_name, e.department_id, d.department_name   
    FROM employees e   
    RIGHT OUTER JOIN departments d   
    ON (e.department_id = d.department_id);   
    等价于   
    SELECT e.last_name, e.department_id, d.department_name   
    FROM employees e, departments d   
    WHERE e.department_id(+)=d.department_id   

结果为：所有员工及对应部门的记录，包括没有任何员工的部门记录。

FULL OUTER JOIN：全外关联

    SELECT e.last_name, e.department_id, d.department_name   
    FROM employees e   
    FULL OUTER JOIN departments d   
    ON (e.department_id = d.department_id);   

结果为：所有员工及对应部门的记录，包括没有对应部门编号department_id的员工记录和没有任何员工的部门记录。