database - Can DB constraits ignore existing records and apply for only new data? -

I want to know the answer for different DB engines but in our case; We have some records that are not unique to any column, and now we want to make that column unique that forces us to remove duplicate values. Do we use Oracle 10G, is it fair? Or is this something like a goto statement? Should we really delete? What if we have millions of records?

You can do this by using the Novadadet enabled hurdle status, but more preferred way to delete.