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: 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!

Re: Theoretical definition for the number of unique values?

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.

Re: Theoretical definition for the number of unique values?

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.

Re: Theoretical definition for the number of unique values?

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.

Re: Theoretical definition for the number of unique values?

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.

Re: Theoretical definition for the number of unique values?

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 --

Re: Theoretical definition for the number of unique values?

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 --

Re: Theoretical definition for the number of unique values?

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

Re: Theoretical definition for the number of unique values?

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

Re: Theoretical definition for the number of unique values?

I found out. It is called "COLUMN CARDINALITY"

Sorry, no prizes.


http://www.informatik.uni-trier.de/~ley/db/journals/tods/Wha ngVT90.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.

Re: Theoretical definition for the number of unique values?

I found out. It is called "COLUMN CARDINALITY"

Sorry, no prizes.


http://www.informatik.uni-trier.de/~ley/db/journals/tods/Wha ngVT90.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.

Re: Theoretical definition for the number of unique values?

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).

Re: Theoretical definition for the number of unique values?

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).

Re: Theoretical definition for the number of unique values?

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".

Re: Theoretical definition for the number of unique values?

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".

Re: Theoretical definition for the number of unique values?

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

>I found out. It is called "COLUMN CARDINALITY"
>
>Sorry, no prizes.
>
>
>http://www.informatik.uni-trier.de/~ley/db/journals/tods/Wh angVT90.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.

Re: Theoretical definition for the number of unique values?

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

>I found out. It is called "COLUMN CARDINALITY"
>
>Sorry, no prizes.
>
>
>http://www.informatik.uni-trier.de/~ley/db/journals/tods/Wh angVT90.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.

Re: Theoretical definition for the number of unique values?

On Apr 27, 8:21 pm, Lemming
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/~ley/db/journals/tods/Wh angVT90.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.

Re: Theoretical definition for the number of unique values?

On Apr 27, 8:21 pm, Lemming
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/~ley/db/journals/tods/Wh angVT90.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.

Re: Theoretical definition for the number of unique values?

On 27 Apr 2007 17:52:00 -0700, hpuxrac
wrote:

>On Apr 27, 8:21 pm, Lemming
>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/~ley/db/journals/tods/Wh angVT90.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.

Mate, most of usenet died more than 2 years ago. What does it matter
if I'm reviving someone's fortnight-old homework? Especially if I am
taking the piss.

Do try to keep up.

Unless, of course, it was *your* homework? Forgive me if so; I can't
be bothered to read back. But I can understand why you might be
feeling a bit sensitive about it.

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

Re: Theoretical definition for the number of unique values?

On 27 Apr 2007 17:52:00 -0700, hpuxrac
wrote:

>On Apr 27, 8:21 pm, Lemming
>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/~ley/db/journals/tods/Wh angVT90.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.

Mate, most of usenet died more than 2 years ago. What does it matter
if I'm reviving someone's fortnight-old homework? Especially if I am
taking the piss.

Do try to keep up.

Unless, of course, it was *your* homework? Forgive me if so; I can't
be bothered to read back. But I can understand why you might be
feeling a bit sensitive about it.

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

Re: Theoretical definition for the number of unique values?

On Apr 27, 9:01 pm, Lemming
wrote:
> On 27 Apr 2007 17:52:00 -0700, hpuxrac
> wrote:
>
>
>
>
>
> >On Apr 27, 8:21 pm, Lemming
> >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/~ley/db/journals/tods/Wh angVT90.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.
>
> Mate, most of usenet died more than 2 years ago. What does it matter
> if I'm reviving someone's fortnight-old homework? Especially if I am
> taking the piss.
>
> Do try to keep up.
>
> Unless, of course, it was *your* homework? Forgive me if so; I can't
> be bothered to read back. But I can understand why you might be
> feeling a bit sensitive about it.

Not exactly.

Different people pick different tools to read these posting.

The cdos group is still very active. This item was cross posted to
various groups and was effectively dead until you chimed in.

Personally I use the google groups interface. If you take a look at
that tool you might have a different opinion about the health of what
used to be usenet. Plus it allows you to see the question from the
op, the replies and the thread in context.

Many of the other people responding in cdos use other tools.

Re: Theoretical definition for the number of unique values?

On Apr 27, 9:01 pm, Lemming
wrote:
> On 27 Apr 2007 17:52:00 -0700, hpuxrac
> wrote:
>
>
>
>
>
> >On Apr 27, 8:21 pm, Lemming
> >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/~ley/db/journals/tods/Wh angVT90.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.
>
> Mate, most of usenet died more than 2 years ago. What does it matter
> if I'm reviving someone's fortnight-old homework? Especially if I am
> taking the piss.
>
> Do try to keep up.
>
> Unless, of course, it was *your* homework? Forgive me if so; I can't
> be bothered to read back. But I can understand why you might be
> feeling a bit sensitive about it.

Not exactly.

Different people pick different tools to read these posting.

The cdos group is still very active. This item was cross posted to
various groups and was effectively dead until you chimed in.

Personally I use the google groups interface. If you take a look at
that tool you might have a different opinion about the health of what
used to be usenet. Plus it allows you to see the question from the
op, the replies and the thread in context.

Many of the other people responding in cdos use other tools.