Update one table with the data in another table

This kind of query is called a correlated sub query. For your requirement, the query would be as below....

update students s
set s.department = (
select e.qualification
from employee e
where s.uniquename = e.uniquename
and s.Homecountry = e.Homecountry
);

---create table and insert statements

create table student(
name varchar2(20),
age  number,
department varchar2(3),
HomeCountry varchar2(10)
);

Table created.

create table employee5(
name varchar2(20),
exp  number,
qualification varchar2(3),
homecountry varchar2(10)
);

Table created.

insert into student values ('Mohan',25,'EEE','India');
insert into student values ('Raja',27,'EEE','India');
insert into student values ('Ahamed',26,'ECE','UK');
insert into student values ('Gokul',25,'IT','USA');
commit;

insert into employee5 values ('Mohan',25,'ECE','India');
insert into employee5 values ('Raja',24,'IT','India');
insert into employee5 values ('Palani',26,'ECE','USA');
insert into employee5 values ('Sathesh',29,'CSE','CANADA');
insert into employee5 values ('Ahamed',28,'ECE','UK');
insert into employee5 values ('Gokul',29,'EEE','USA');
commit;

Before updating the data...

SQL> select * from student;

NAME                        AGE DEP HOMECOUNTR
-------------------- ---------- --- ----------
Mohan                        25 EEE India
Raja                         27 EEE India
Ahamed                       26 ECE UK
Gokul                        25 IT  USA

SQL> select * from employee5;

NAME                        EXP QUA HOMECOUNTR
-------------------- ---------- --- ----------
Mohan                        25 ECE India
Raja                         24 IT  India
Palani                       26 ECE USA
Sathesh                      29 CSE CANADA
Ahamed                       28 ECE UK
Gokul                        29 EEE USA

Update statement and results

1  update student s set s.age =
2     ( select e.exp
3          from employee5 e
4          where e.name = s.name
5            and e.homecountry = s.homecountry
6*    )
SQL> /

4 rows updated.

SQL> select * from student;

NAME                        AGE DEP HOMECOUNTR
-------------------- ---------- --- ----------
Mohan                        25 EEE India
Raja                         24 EEE India
Ahamed                       28 ECE UK
Gokul                        29 IT  USA

SQL> commit;

Commit complete.

update student s
set s.age = (select e.exp
from employee5 e
where e.name = s.name
and e.homecountry = s.homecountry
and rownum < 2
)
where s.age in (select age from employee5)

It wont show the message subquery returns more than one record