This is correct by spec. NULLs are a special case.
From UNIQUE <table subquery) which unique constraints are definedagainst:
2) If there are no two rows in T such that the value of each column in one row is non-null and is equal to the value
ofthe cor- responding column in the other row according to Subclause 8.2, "<comparison predicate>", then the result
ofthe <unique predi- cate> is true; otherwise, the result of the <unique predicate> is false.
[This means that there will be no two rows such that the value
of each column in non-null and is equal to to the value in the
other since one of the columns is null]
On Sun, 13 May 2001, Domingo Alvarez Duarte wrote:
> when creating a index unique in a table that accept nulls the unique
> constraint doesn't work.
>
> Example:
> ---
> create table test_unique(i1 integer, i2 integer, unique(i1,i2);
> insert into test_unique(1,null);
> insert into test_unique(1,null);
> insert into test_unique(1,null);
> ---
> all "inserts" above insert sucefully.
>
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