Theoretical definition for the number of unique values?

[sqlservernewbie]

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!

[Bob Badour]

sq************* @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.

[Aloha Kakuikanu]

On Apr 12, 7:19 am, sqlservernew... @yahoo.com wrote:

"the number of unique values in a column?"

NDV - number of distinct values. There is nothing theoretical about it.

[Mark D Powell]

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 --

[fitzjarrell]

On Apr 12, 10:19 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!

I believe it is referred to as 'cardinality'. Which should be covered
in your text and by your instructor.
David Fitzjarrell

[sqlservernewbie]

I found out. It is called "COLUMN CARDINALITY"

Sorry, no prizes.
http://www.informatik.uni-trier.de/~...WhangVT90.html
(1) obtaining the column cardinality (the number of unique values in a
column of a relation) and
(2) obtaining the join selectivity (the number of unique values in the
join column resulting from an unconditional join divided by the number
of unique join column values in the relation to Be joined).

These two parameters are important statistics that are used in
relational query optimization and physical database design.

http://www.idig.za.net/mysqlindexes/2006/11/09/
Column cardinality. This is the number of unique values contained in a
column. Indexes work best when there is a high cardinality. Put
another way, the more unique values there are (fewer duplicates) the
better that column will be for indexing. Consider the ID number column
of the previous example. Here there are no duplicates, only unique
values. This column will be ideal for indexing. On the other end of
the scale may be the first names column. Here there will probably be a
number of duplicate names (fewer unique values) and a lower
cardinality compared to the ID column.

[Cimode]

On Apr 12, 4:19 pm, 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!

I do not believe there is a specific terminology or usefulness for
such concept. You may call it *domain attribute projection
cardinality* (I just made it up but it could be a description of
underlying concepts).

[William Robertson]

On Apr 12, 4:19 pm, sqlservernew... @yahoo.com wrote:

Degree
the number of attributes a relation contains
Number of fields in a table

A table does not have "fields".

[Lemming]

On 12 Apr 2007 18:23:07 -0700, sq************* @yahoo.com wrote:

>I found out. It is called "COLUMN CARDINALITY"

Sorry, no prizes.
http://www.informatik.uni-trier.de/~...WhangVT90.html
(1) obtaining the column cardinality (the number of unique values in a
column of a relation) and
(2) obtaining the join selectivity (the number of unique values in the
join column resulting from an unconditional join divided by the number
of unique join column values in the relation to Be joined).

These two parameters are important statistics that are used in
relational query optimization and physical database design.

http://www.idig.za.net/mysqlindexes/2006/11/09/
Column cardinality. This is the number of unique values contained in a
column. Indexes work best when there is a high cardinality. Put
another way, the more unique values there are (fewer duplicates) the
better that column will be for indexing. Consider the ID number column
of the previous example. Here there are no duplicates, only unique
values. This column will be ideal for indexing. On the other end of
the scale may be the first names column. Here there will probably be a
number of duplicate names (fewer unique values) and a lower
cardinality compared to the ID column.

Yes, cardinality is the correct term.

Now, for bonus credits: can anyone tell me the correct term for
someone who posts a homework question here, gets an answer, and then
pretends he worked the answer out for himself?

Lemming
--
Curiosity *may* have killed Schrodinger's cat.

[hpuxrac]

On Apr 27, 8:21 pm, Lemming <thiswillbou... @bumblbee.demon .co.uk>
wrote:

On 12 Apr 2007 18:23:07 -0700, sqlservernew... @yahoo.com wrote:

I found out. It is called "COLUMN CARDINALITY"

Sorry, no prizes.

http://www.informatik.uni-trier.de/~...WhangVT90.html

(1) obtaining the column cardinality (the number of unique values in a
column of a relation) and
(2) obtaining the join selectivity (the number of unique values in the
join column resulting from an unconditional join divided by the number
of unique join column values in the relation to Be joined).

These two parameters are important statistics that are used in
relational query optimization and physical database design.

http://www.idig.za.net/mysqlindexes/2006/11/09/

Column cardinality. This is the number of unique values contained in a
column. Indexes work best when there is a high cardinality. Put
another way, the more unique values there are (fewer duplicates) the
better that column will be for indexing. Consider the ID number column
of the previous example. Here there are no duplicates, only unique
values. This column will be ideal for indexing. On the other end of
the scale may be the first names column. Here there will probably be a
number of duplicate names (fewer unique values) and a lower
cardinality compared to the ID column.

Yes, cardinality is the correct term.

Now, for bonus credits: can anyone tell me the correct term for
someone who posts a homework question here, gets an answer, and then
pretends he worked the answer out for himself?

Lemming
--
Curiosity *may* have killed Schrodinger's cat

Way to jump all over a thread that died 2 weeks ago.