用in如何就出现一行数据_Oracle管理

用in怎么就出现一行数据？

SQL code

insert into deptment(dept_id,dept_name)values('1','数学');insert into deptment(dept_id,dept_name)values('2','计算机');insert into deptment(dept_id,dept_name)values('3','中文');insert into deptment(dept_id,dept_name)values('4','经济学');insert into class_table(class_id,subject_name,dept_name,entertime,person_num)values('101','软件','计算机','1995',20);insert into class_table(class_id,subject_name,dept_name,entertime,person_num)values('102','微电子','计算机','1996',30);insert into class_table(class_id,subject_name,dept_name,entertime,person_num)values('111','无机化学','化学','1995',29);insert into class_table(class_id,subject_name,dept_name,entertime,person_num)values('112','高分子化学','化学','1996',25);insert into class_table(class_id,subject_name,dept_name,entertime,person_num)values('121','统计','数学','1995',20);insert into class_table(class_id,subject_name,dept_name,entertime,person_num)values('141','国际贸易','金融学','1997',30);insert into class_table(class_id,subject_name,dept_name,entertime,person_num)values('131','现代汉语','中文','1996',20);insert into class_table(class_id,subject_name,dept_name,entertime,person_num)values('151','国际汉语','金融学','1996',14);

要求如下:
列出所有人数大于等于28的系的编号和名称

我的解法：

SQL code

 select deptment.dept_id,deptment.dept_name from deptment where deptment.dept_name in(select a.dept_name from class_table a where deptment.dept_name = a.dept_namegroup by a.dept_name having sum(person_num)>28);

但是只出现一个计算机系，在子查询里面都有三个的，不知道是为什么？不用in的话怎么解？

------解决方案--------------------
去掉where deptment.dept_name = a.dept_name.

SQL code

select deptment.dept_id,deptment.dept_name from deptment where deptment.dept_name in(select a.dept_name from class_table agroup by a.dept_name having sum(person_num)>28);
------解决方案--------------------
还有,depment表里面没有‘化学’，‘金融学’