This doesn't apply to just Oracle. It is a
consideration in other databases I understand.

Tom Stillman

--- "Dereck L. Dietz" <dietzdl@...> wrote:

> From the standpoint of an Oracle programmer/dba
> there is a difference between a 0 and a NULL.
>
> A zero is a value. It's value is ZERO.
>
> A null by definition is an ABSENCE of a value.
>
> If you're not careful with the above distinctions
> you could default a value to 0 when it should
> actually remain a NULL and by doing so throw off
> statistics.
>
> For example a table with 10 rows of values:
>
> In the first - 5 values defaulted to 0 and 5 values
> with actual values. The average of the values in
> table is 1.5
>
> 0
> 0
> 0
> 0
> 0
> 1
> 2
> 3
> 4
> 5
>
> In the second table - 5 values NOT defaulted to 0
> and the same 5 actual values as the first. The
> average of the values in this table is 3.
>
> NULL
> NULL
> NULL
> NULL
> NULL
> 1
> 2
> 3
> 4
> 5
>
> The difference is in the first table the average
> function AVG() is using ALL the values even the 0
> values. In the second the NULL values don't exist
> so the AVG() function is only using the 5 values it
> knows about.




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