use tempdb

create table target (
	a int unique clustered
	)
insert target 
        select 1 union all 
        select null

create table pointer (
        a int references target(a) on delete cascade,
        b int,
        c int,
        constraint uq_pointer unique clustered (a,b,c)
        )
insert pointer 
        select 1,2,3 union all 
        select 1,2,null union all
        select 1,null,3 union all 
        select null,2,3 union all
        select null,null,3 union all
        select null,2,null union all 
        select 1,null,null union all
        select null,null,null 

select * from pointer  -- returns 8 rows

insert pointer select null,null,null -- this statement fails demonstrating that a
                                            -- unique constraint requires only absence of equality, 
                                            -- not presence of inequality

drop table pointer, target