求MySQL语句写法

这个是建2个表的语句，一个是人员表，一个职位表，求出职位空缺数，写到职位表的SPACE里
SQL code

INSERT INTO job_message
   (`position_id`, `position_name`, `total`, `level`, `space`, `department`, `department_id`, `detail`, `found_date`, `del`)
VALUES
   (9, 'a', 20, '', 20, 'x', 1, '', '2011-08-03 09:42:26', 1);

INSERT INTO job_message
   (`position_id`, `position_name`, `total`, `level`, `space`, `department`, `department_id`, `detail`, `found_date`, `del`)
VALUES
   (10, 'xx', 12, '', 12, 'y', 2, '', '2011-08-03 09:44:26', 1);

INSERT INTO job_message
   (`position_id`, `position_name`, `total`, `level`, `space`, `department`, `department_id`, `detail`, `found_date`, `del`)
VALUES
   (11, '4部1分', 2, '', 2, '4部', 4, '', '2011-08-03 11:56:34', 1);

第二个表
SQL code

INSERT INTO employee_info
   (`user_id`, `password`, `name`, `sex`, `department`, `position_id`, `position`, `office_number`, `mail`, `birthday`, `ismarry`, `telephone`, `postcode`, `address`, `id_card`, `found_date`, `del`, `budget`, `submit`, `sys`, `formInfo`)
VALUES
   (58, '', '', NULL, 'y', 10, 'xx', '', '', '', NULL, '', '', '', '', '2011-08-03 09:44:38', 1, NULL, NULL, NULL, NULL);

INSERT INTO employee_info
   (`user_id`, `password`, `name`, `sex`, `department`, `position_id`, `position`, `office_number`, `mail`, `birthday`, `ismarry`, `telephone`, `postcode`, `address`, `id_card`, `found_date`, `del`, `budget`, `submit`, `sys`, `formInfo`)
VALUES
   (59, '', '', NULL, 'y', 10, 'xx', '', '', '', NULL, '', '', '', '', '2011-08-03 09:45:01', 1, NULL, NULL, NULL, NULL);

INSERT INTO employee_info
   (`user_id`, `password`, `name`, `sex`, `department`, `position_id`, `position`, `office_number`, `mail`, `birthday`, `ismarry`, `telephone`, `postcode`, `address`, `id_card`, `found_date`, `del`, `budget`, `submit`, `sys`, `formInfo`)
VALUES
   (60, '', '', NULL, 'y', 10, 'xx', '', '', '', NULL, '', '', '', '', '2011-08-03 09:45:24', 1, NULL, NULL, NULL, NULL);

INSERT INTO employee_info
   (`user_id`, `password`, `name`, `sex`, `department`, `position_id`, `position`, `office_number`, `mail`, `birthday`, `ismarry`, `telephone`, `postcode`, `address`, `id_card`, `found_date`, `del`, `budget`, `submit`, `sys`, `formInfo`)
VALUES
   (65, '', '', NULL, 'x', 9, 'a', '', '', '', NULL, '', '', '', '', '2011-08-03 10:48:55', 1, NULL, NULL, NULL, NULL);

这个是我写的执行语句，没有效果。。

SQL code

update job_message,employee_info, (
select  count(employee_info.position_id) as employeed ,position_name from employee_info
  right join job_message on  job_message.position_id =  employee_info.position_id  where job_message.del=1 
and job_message.del=1
group by employee_info.position_id ) as p set job_message.space=job_message.total-case when p.employeed is null then 0 else p.employeed end  where job_message.position_id =  employee_info.position_id and job_message.del=1;

分步来吧 一步步看结果

有空值，group by 排序有问题，我加了个ORDER BY 之后结果有变化了，但是第二条数据和第一条变化一样。
SQL code

update job_message,employee_info, (
select  count(employee_info.position_id) as employeed ,position_name from employee_info
  right join job_message on  job_message.position_id =  employee_info.position_id  where job_message.del=1 
and job_message.del=1
group by employee_info.position_id order by  isnull(employee_info.position_id) ,employee_info.position_id  ) as p set job_message.space=job_message.total-(case when p.employeed is null then 0 else p.employeed end ) where job_message.position_id =  employee_info.position_id and job_message.del=1;