Unique constraint

I am creating one new table using parallels by joining 3more table using union..here I have 2 doubts 1st is I want only unique data into my new table can I use unique constraint if yes how?? And second one is where can I place that constraint before or after parallels command...


Create table temp_unq nologging parallel (degree 16) as
Select id from temp1
 select id from temp2
Select id from temp3;

Here I can't do like first creating table with unique constraint on columns then insertion of data because it's time taking to insert, for creation I can use parallels.