Theoretical definition for the number of unique values? - MySQL

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 ...

1. Theoretical definition for the number of unique values?

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: 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!

sqlservernewbie@yahoo.com Guest

2. Re: Theoretical definition for the number of unique values?

com wrote:

Using your definitions, it'd probably be "the cardinality of the
domain."

Larry Coon
University of California
Larry Guest

3. Re: Theoretical definition for the number of unique values?

On Apr 12, 11:47 am, com wrote:

distinct values

For a column or set of columns which together an index is created from
you can think of it as distinct keys

hpuxrac Guest

4. Re: Theoretical definition for the number of unique values?

com wrote:

(Column) Cardinality = number of distinct column/attribute values.
Table Cardinality = number of rows in a table.

--
Jeroen

The Guest

5. Re: Theoretical definition for the number of unique values?

In article <461e877b\$0\$328\$xs4all.nl>,
>
> (Column) Cardinality = number of distinct column/attribute values.
> Table Cardinality = number of rows in a table.[/ref]

Shouldn't that be *distinct* (non-duplicate) rows in the table?

--
Remove the ns_ from if replying by e-mail (but keep posts in the
newsgroups if possible).
David Guest

6. Re: Theoretical definition for the number of unique values?

On Apr 12, 4:14 pm, David Kerber
<com> wrote: [/ref]
> [/ref]
> [/ref]
> [/ref]
> [/ref]
> [/ref]
> [/ref]
> [/ref]
> [/ref]
> [/ref]
> [/ref]
> [/ref]
> [/ref]
> [/ref]
> [/ref]

>
> Shouldn't that be *distinct* (non-duplicate) rows in the table?[/ref]

I believe that one of the cardinal rules (pun intended) of RDBMS
theory is that a table can never have duplicate rows.

Gord Guest

7. Re: Theoretical definition for the number of unique values?

> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> > Shouldn't that be *distinct* (non-duplicate) rows in the table?[/ref]
>
> I believe that one of the cardinal rules (pun intended) of RDBMS
> theory is that a table can never have duplicate rows.[/ref]

True. There's no point in having duplicate rows, cause you can't tell
which one you're handling :-)

--
Martijn Tonies
Database Workbench - development tool for MySQL, and more!
Upscene Productions
http://www.upscene.com
My thoughts:
http://blog.upscene.com/martijn/
Database development questions? Check the forum!
http://www.databasedevelopmentforum.com

Martijn Guest

8. Re: Theoretical definition for the number of unique values?

In article <461f2fcd\$0\$11982\$news.xs4all.nl>,
removethis.com says...
> >
> > I believe that one of the cardinal rules (pun intended) of RDBMS
> > theory is that a table can never have duplicate rows.[/ref]
>
> True. There's no point in having duplicate rows, cause you can't tell
> which one you're handling :-)[/ref]

True, but are you telling me you've never had it happen accidentally??

--
Remove the ns_ from if replying by e-mail (but keep posts in the
newsgroups if possible).
David Guest

9. Re: Theoretical definition for the number of unique values?

David Kerber wrote:

>
> True, but are you telling me you've never had it happen accidentally??[/ref]

Assuming the database is being used as more than an electronic form
of Excel or 3x5 cards ... no. Primary key constraints were invented
for a reason. Like seat belts they are most useful when used.
--
Daniel A. Morgan
University of Washington
damorganx.washington.edu
(replace x with u to respond)
Puget Sound Oracle Users Group
www.psoug.org
DA Guest

Posting Permissions

• You may not post new threads
• You may not post replies
• You may not post attachments
• You may not edit your posts
•