Re: Theoretical definition for the number of unique values?

On 27 Apr 2007 17:52:00 -0700, hpuxrac <johnbhurley_at_sbcglobal.net> wrote:

>On Apr 27, 8:21 pm, Lemming <thiswillbou..._at_bumblbee.demon.co.uk>
>wrote:
>> On 12 Apr 2007 18:23:07 -0700, sqlservernew..._at_yahoo.com wrote:
>>
>>
>>
>>
>>
>> >I found out. It is called "COLUMN CARDINALITY"
>>
>> >Sorry, no prizes.
>>
>> >http://www.informatik.uni-trier.de/~ley/db/journals/tods/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.

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.