Re: Theoretical definition for the number of unique values?

From: Bob Badour <bbadour_at_pei.sympatico.ca>
Date: Thu, 12 Apr 2007 15:45:26 GMT
Message-ID: <qusTh.23687$PV3.235507@ursa-nb00s0.nbnet.nb.ca>

sqlservernewbie_at_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?

It is the cardinality of the projection onto the attribute, which may or may not equal the cardinality of the domain. Same concept just qualified differently. Received on Thu Apr 12 2007 - 10:45:26 CDT