请大家帮忙看看:我的update语句为什么不能更新ok数据表?
senior表:
senior_exam_num char(50) not null
senior_name char(50) not null
sex char(4)
senior_num char(50) not null key
ok表:
senior_exam_num char(50)
senior_name char(50) not null
sex char(4)
senior_num char(50) not null key
......
select * from senior where senior_num=02331241030; //这样的语句可以查询到结果
但是
update ok
set senior_exam_num = (select senior_exam_num from senior where senior_num = ok.senior_num)
where senior_exam_num=0; //ok表的senior_exam_num先可被更新为0;但是用senior表的senior_exam_num来更新它却得到null。请问是为什么?