Re: why NOT NULL in PRIMARY key??

[Dawid Kuroczko]

On 4/26/05, Jay Blanchard <jay.blanchard@stripped> wrote:
> [snip]
> The same is true for any other value... Now that the columns have a NOT
> NULL
> constraint the records that previously contained NULL now hold '0'.
> 
> x y
> x 0
> x z
> x 0
> 
> Now, how do you uniquely identify the 2nd and 4th rows?
> [/snip]
> 
> The database would have thrown an error when you tried to create row 4.

No.  NULL is not a value.  It is a lack of value.  Kind of a special
symbol, like infinity in mathematics.  You cannot say that
infinity = infinity or infinity <> infinity.  Same as you cannot
say that NULL = NULL or NULL <> NULL.

  Regards,
       Dawid