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Posted by Mark D Powell on 04/12/07 18:09
On Apr 12, 11:19 am, sqlservernew...@yahoo.com wrote:
> Hi Everyone,
>
> Here is a theoretical, and definition question for you.
>
> In databases, we have:
>
> Relation
> a table with columns and rows
>
> Attribute
> a named column/field of a relation
>
> Domain
> a set of allowable values for one or more attributes
>
> Tuple
> a row of a relation
>
> Degree
> the number of attributes a relation contains
> Number of fields in a table
>
> Cardinality
> the number of tuples/rows a relation contains
>
> But!
>
> What is the definition for the number of unique values in a field?
>
> So, if you have 100 rows in a table, and the field is
> the gender field, with only values of: Y, N.
> You have 2 unique values.
>
> What do we call this concept?
> "the number of unique values in a column?"
>
> Is there one?
>
> Thanks a lot!
The Oracle statistics refer to this as the number of DISTINCT values.
Off the top of my head I do not remember any relational theory concept
that applies. The range of valid values for the attribute would be
the DOMAIN and each value in the domain would be distinct since the
domain concept has no relation to the actual number of occurrences for
real data.
Maybe someone else will remember a concept that applies to one of the
versions of relational theory.
HTH -- Mark D Powell --
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