From: <beacker@stripped>
> create table a (
> b int not null,
> c int null
> primary_key(b,c)
> );
>
> With values:
> 1 null
> 1 null
>
> Logically these are unique records under the standard proviso that
> null != null. Yet how could I uniquely identify the first row to delete
> that row?
First of all NULL is neither equal nor not-equal to NULL. The outcome of
comparing NULL to NULL is unknown AKA 'NULL' ;-)
Secondly, the previous line is only true for comparing values within
queries. If it comes to UNIQUE indexes than most of the time NULL != NULL,
but in the case of BDB tables NULL = NULL (only a single NULL entry can be
present in the UNIQUE index of a BDB table).
While BDB tables are not the role model for other table types I used this
example to indicate that MySQL is capable of handling NULLs this way.
Thirdly, if NULL=NULL for such an index then the secod row could not have
been inserted in the first place.
Regards, Jigal.
