Re: Design question : Nullable FK & FK To Unique Key

"""krislioe_at_gmail.com ΠΙΣΑΜ(Α):
"""
> Hi gurus,
>
> Our Invoice may or may not has Orders before it. User can input order
> and produce invoice from it, OR they can just input Invoice without
> order. So we plan to have Nullable FK on Invoice.
>
> The question is :
>
> 1) What is the bad side of Nullable FK ?
>
> 2) FK from Invoice to Orders , not refering to PK , but to the Unique
> Key of the Orders, what is the bad side of this ?
>
> Thank you for your help,
> xtanto

1. Generally the same as having index on nullable columns.
2. Why? Why do you use unique constraint in place of PK?