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Tekapo
Neurotic (0) Inmate
Newly admitted

From:
Insane since: Aug 2004

posted posted 08-12-2004 03:39

Hi, guys. May I ask you something, please?
I sometimes design table to create business apprecations.
I am newbie of this industory.
I sometimes found tables which does not include primary key.
I wonder when I don't need the primary key.
Any your comments about this will be highly appreciated.
Thanks.

MalFunkShun
Maniac (V) Inmate

From: few miles outside philly
Insane since: Jul 2001

posted posted 08-12-2004 04:54

You wouldn't need a primary key if the table exsist by itself and/or if it is never referenced or shared data from another table.

Tekapo
Obsessive-Compulsive (I) Inmate

From:
Insane since: Aug 2004

posted posted 08-12-2004 06:15

Hi, Mal. Thanks for that.
But the last bit of your comments.

quote:
if it is never referenced or shared data from another table.



Do you mean that Table has to have the PK when I want to join the table???
And why is that???

bitdamaged
Maniac (V) Mad Scientist

From: 100101010011 <-- right about here
Insane since: Mar 2000

posted posted 08-12-2004 06:21

Primary keys are unique identifiers for a row in the table. If you don't have one, when you do a join you don't have a unique way of identifying a particular row in the table.

Another place you usually won't have one is relational tables which are usually just rows of Primary Keys. For example if you have a table of students and a table of classes then a third table of students and classes which only contains the Primary Key for a student and a the primary key of a class. You'd use the student table to get the student info, the class table to get the class info and the student/class table to find out which students are in which classes.



.:[ Never resist a perfect moment ]:.

Tekapo
Obsessive-Compulsive (I) Inmate

From:
Insane since: Aug 2004

posted posted 08-12-2004 07:00

Thanks a lot but let me say a wee bit more, BD.
So when I do a join I will not need the primary key only when each table's row has absolute unique data, right?
Thanks.

butcher
Paranoid (IV) Inmate

From: New Jersey, USA
Insane since: Oct 2000

posted posted 08-12-2004 14:14

The absolute unique data is what you would declair the primary key.

In bitdamaged's example, the first 2 tables he mentioned (students and classes) would both have a field that was a primary key. This key would quite often be an auto increment ID column. The table that wouldn't need a primary key would be the third table because this table would be used to relate the fisrt 2 tables together. It would probably hold just 2 fields 'students' and 'classes' and would hold an ID from each of the other 2 tables and be used in queries to relate the other 2 tables together.

I hope that helped.

-Butcher-

MalFunkShun
Maniac (V) Inmate

From: few miles outside philly
Insane since: Jul 2001

posted posted 08-12-2004 20:11

Wow... I just had my DB class last semester. I wish you had the teacher I did. He pounded all this info into us and the book that went along with it was great. Before you start rambling off questions and expecting easy answers, can I ask if you have sat down and read a book yourself about relational databases. This may be your best solution and provide answers in a more orderly fashion. I don't mean to say this to be rude but I don't want to give you my few thousand dollars in education in a few threads. If I had to suffer for 6+ months to learn this then you should at least do some reading before taking the easy way out. Or at least tell me a specific issue where this is coming up for you.

Also, setting a primary key is not a big task. What would be your reasoning for not wanting one?



(Edited by MalFunkShun on 08-12-2004 20:11)

bitdamaged
Maniac (V) Mad Scientist

From: 100101010011 <-- right about here
Insane since: Mar 2000

posted posted 08-12-2004 21:39

You don't need primary keys, you want primary keys. They are the identifiers used to make a row in a table unique. When you do a join you are going to get a SQL result set with a number of rows. Without using a primary key there's nothing to distinguish the rows so you're just going to have a chunk of data that you can't really do much with.



.:[ Never resist a perfect moment ]:.

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