DB2 : everything about UDB universe

One of the junior DBAs asked me this question. I thought it was a good enough reason to blog.

I am not touching what primary key helps us achieve, but what's difference in executing just

create unique index x.y on table xx.yy allow reverse scans;

than

create unique index x.y on table xx.yy allow reverse scans;
alter table xx.yy add constraint -- and give the columns in the above defined unique index

unique index can have one row with null value in either of columns, while primary key disallows it :)