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PRIMARY KEY VS UNIQUE KEY*


PRIMARY KEY VS UNIQUE KEY*

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Kazarf
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Hi everyone,

Does anybody know any cases/example that we need to use UNIQUE KEY instead of PRIMARY KEY? I understand that UNIQUE KEY can allow one NULL value in the data, but I don't know the reason for that. Why do we need that extra NULL VALUE? And, When do we need it?

Thanks!
Kaz
PaulB-TheOneAndOnly
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Kazarf (7/29/2013)
Does anybody know any cases/example that we need to use UNIQUE KEY instead of PRIMARY KEY? I understand that UNIQUE KEY can allow one NULL value in the data, but I don't know the reason for that. Why do we need that extra NULL VALUE? And, When do we need it?


A Primary Key is a Referential Integrity constraint, that's why it should never allow a Null value.

On the other hand, an Unique Index is an index that doesn't allow for duplicate values but it is not a Referential Integrity constraint.

In my opinion, you do not "need" and extra Null Value - it is just allowed to be there even if I would never ever allow for Null Values on a Unique Index, it is not elegant and a symptom that something is wrong with the way data was modeled.

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Pablo (Paul) Berzukov

Author of Understanding Database Administration available at Amazon and other bookstores.

Disclaimer: Advice is provided to the best of my knowledge but no implicit or explicit warranties are provided. Since the advisor explicitly encourages testing any and all suggestions on a test non-production environment advisor should not held liable or responsible for any actions taken based on the given advice.
Kazarf
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Thank You so much! ^^
kapil_kk
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Good one Paul :-)

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You can also create multiple unique indexes on the same table, but you can have only one primary key.
A unique index can also be filtered, while a PK cannot.



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Koen Verbeeck (7/30/2013)
You can also create multiple unique indexes on the same table, but you can have only one primary key.
A unique index can also be filtered, while a PK cannot.


I am not clear with this statement_

A unique index can also be filtered, while a PK cannot

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http://msdn.microsoft.com/en-us/library/cc280372%28v=sql.100%29.aspx


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Thanks Gail for the useful link :-)

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On the other hand, an Unique Index is an index that doesn't allow for duplicate values but it is not a Referential Integrity constraint.

Paul,what exactly you mean by that..

As we can have referential integrity constraint column referring on unique key columns also...

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psingla (7/30/2013)

On the other hand, an Unique Index is an index that doesn't allow for duplicate values but it is not a Referential Integrity constraint.

Paul,what exactly you mean by that..

As we can have referential integrity constraint column referring on unique key columns also...


Yes, but then you are creating a foreign key on top of some columns who happen to be part of unique index. The index itself has nothing to do with referential integrity.



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