Instead of =, use In
I think your query will be looking something like
select * from employee where empid=(select empid from employee where dept='SE')
So now what happens, the subquery will return you more than one row but since you mentioned condition'=', it returns error.
In that case, you have to use IN or exists
select * from employee where empid in (select empid from employee where dept='SE')
In case if you used !=, then use not in.
But note: Not in will affect performance use not exists