When I write this query in sql plus show me error (ORA-00998: must name this expression with a column alias)
create or replace view vw_salary as select dname ,
(select count(*) from employee where dno=department.dnumber) from department;
Columns in a view need to have a name. So, you need
as after the subquery:
create or replace view vw_salary as select dname, (select count(*) from employee e where e.dno = d.dnumber ) as NumEmployees from department d;
I strongly encourage you to use table aliases and qualified column names (that is use the table alias). This is particularly important for correlated subqueries, where it is easy to make a mistake and that is hard to debug.
I also note that the view is called
vw_salary, but there is no salary information.