Posted by Bob Badour on 04/12/07 15:45

sqlservernewbie@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.

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