Finding whether duplicates exists in 2 select statements

Could you please help me with SQL statement to find duplicate names present in first Select statement and the second.

For example : I need to find whether student with same name in Class 1 is present or not in Class 2 or 3.

select firstname from College where classid = 1

and Second select statement is :

select firstname from College where classid in (2, 3)

If there is matching occurrences for any name then I want to select that record.

Answer

Join could be one option

select a.firstname
from
(
select firstname from College where classid = 1
)a join (
select firstname from College where classid in(2,3)
)b on a.firstname=b.firstname

OR alternatively, you can try using exists

select firstname from College c where classid = 1
and exists (select 1 from College c1 where c.firstname=c1.firstname and classid in(2,3))