Oracle trigger creation

oracle Create a trigger , There is a table with the following structure

What I want to ask is if I insert a new piece of data ,dept_name For Nanjing , Do not insert dept_code Under the circumstances , according to dept_name, Add... To him 320110
Do with trigger , Execute every row inserted
The code is as follows
create table ceshi (id number,name VARCHAR2(50),dept_name VARCHAR2(50),dept_code VARCHAR2(50))
insert into ceshi values(1,'zhangsan',' Beijing ','110000');
insert into ceshi values(2,'zhangsan',' Shanghai ','310000');
insert into ceshi values(3,'zhangsan',' nanjing ','320100');
insert into ceshi values(4,'zhangsan',' Shenzhen ','440300');
insert into ceshi values(5,'zhangsan',' Guangzhou ','440100');
commit;
select * from ceshi