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Posted by David Kerber on 04/13/07 12:19
In article <461f2fcd$0$11982$e4fe514c@dreader14.news.xs4all.nl>,
m.tonies@upscene.removethis.com says...
>
> > > > > 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: M, F.
> > > > > The result is 2 unique values.
> > >
> > > > > What do we call this concept?
> > > > > "the number of unique values in a column?"
> > >
> > > > > Is there one?
> > >
> > > > > Thanks a lot!
> > >
> > > > (Column) Cardinality = number of distinct column/attribute values.
> > > > Table Cardinality = number of rows in a table.
> > >
> > > Shouldn't that be *distinct* (non-duplicate) rows in the table?
> >
> > I believe that one of the cardinal rules (pun intended) of RDBMS
> > theory is that a table can never have duplicate rows.
>
> True. There's no point in having duplicate rows, cause you can't tell
> which one you're handling :-)
True, but are you telling me you've never had it happen accidentally??
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