Primary Key on a View
Your code creates a column with the name PRIMARY_KEY, but this is not the same as a primary key constraint. (It is not enough to be unique.) As Branko Dimitrijevic pointed out only logical constraints can be defined on views, and a primary key constraint will be enforced by Oracle.

On a materialized view where you store your data physically in the database a index may help you to find your data faster. A primary key constraint may be helpful if you want to use replication. But a materialized view is more of a table than a view.

I hope this helps to answer your questions.