Re: Is this JOIN good ?

From: Brian Peasland <dba_at_nospam.peasland.net>
Date: Mon, 16 Oct 2006 23:13:26 GMT
Message-ID: <J794IK.8G9@igsrsparc2.er.usgs.gov>

klabu wrote:

>> Define a "complete" join, please.
>>
>> -- 
>> David Portas

>
>
> By "complete" I meant : "sound/correct" i.e. NON-CARTESIAN
>
> I'm beginning to see that there definitely need s to be more predicates for
> the result to be "sound/correct"
>
>

Basically, there are four types of joins, CARTESIAN (sometimes called a CROSS JOIN), EQUIJOIN, INNER, and OUTER. The OUTER join can be further classified as LEFT OUTER, RIGHT OUTER, and FULL OUTER.

What you originally wrote can be classified as an INNER join.

For more information, please see the following:

http://en.wikipedia.org/wiki/Inner_join

I have never heard of a "complete join" before. The closest I have come to is that the selection, projection, and join operators are part of a *complete* set of operations when pertaining to relational algebra. The term *complete* here means that all other operations can be formed out of this set of operations.

Whether or not the SQL join you wrote is "correct" all depends on your table structure, and the information you are trying to get out of that table.

HTH,
Brian

-- 
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Brian Peasland
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Received on Mon Oct 16 2006 - 18:13:26 CDT