Update error with third subquery

Question

create table department(dept_id number(10), dept_name varchar2(100));
insert into department values(1, 'Administration');
insert into department values(3, 'Marketing');
commit;
create table employee(
    emp_id number(10), emp_name varchar2(200), birthday date, dept_id number(10)
insert into employee values(1, 'Jonh', to_date('02/03/1982', 'dd/mm/yyyy'), 1);
insert into employee values(2, 'Anna', to_date('27/09/1980', 'dd/mm/yyyy'), 1);
insert into employee values(3, 'Jerry', to_date('19/10/1984', 'dd/mm/yyyy'), 1);
insert into employee values(4, 'Tom', to_date('21/06/1985', 'dd/mm/yyyy'), 2);
insert into employee values(5, 'Lee', to_date('04/09/1979', 'dd/mm/yyyy'), 2);
insert into employee values(6, 'Jin', to_date('12/12/1980', 'dd/mm/yyyy'), 2);
insert into employee values(7, 'Marry', to_date('20/11/1983', 'dd/mm/yyyy'), 2);
commit;
--Now I must add a new column to contain the oldest employee for each department.
alter table department add oldest_employee number(10);
update department T1
set oldest_employee = (
        select emp_id
        from (
                select T2.emp_id, row_number() over(order by t2.birthday desc) as recno
                from employee T2
                where T2.dept_id = T1.dept_id)
        where rownum = 1
commit;
Error: ORA-00904: "T1"."DEPT_ID": invalid identifier
I don't know why. Pls help me.
Edited by: user4583682 on Apr 23, 2009 2:41 AM

Answer 1

Thanks SeánMacGC so much.
Although there are some errors, I have already understood your mean. I post the exact solution again.
update department T1
set oldest_employee = (
        select T2.emp_id
        from employee T2
        where T2.dept_id = T1.dept_id
            and T2.birthday = (
                    select min(t3.birthday)
                    from employee T3
                    where T2.dept_id = T3.dept_id
    );Edited by: user4583682 on Apr 23, 2009 3:29 AM
Edited by: user4583682 on Apr 23, 2009 3:29 AM