oracle exists用法解析

     exists用来检查子查询返回行的存在性。通常我们并不关心子查询返回多少行，只关心子查询是否有任何行被返回；

     select employee_id,last_name from employees out 

     where exists

           (select employee_id from employees inner where inner.manager_id = out.employee_id)

     order by employee_id

     等价于

    select employee_id,last_name from employees out 

           where exists

                     (select employee_id from employees inner where inner.manager_id = out.employee_id)

     order by employee_id