Re: URGENT: Primary key vs Unique Index

In article <33133DFD.6162_at_mediadata.com.sg> mars_at_mediadata.com.sg writes:
>Bill Bearden wrote:
>>
>> There are two important differences between Primary and Unique Keys.
>>
>> 1. There can only be one Primary Key on a table. There can be more than
>> one column with Unique Key.
>> 2. Primary Key columns can't contain NULL. Unique Key columns can.
>>
>
>Pardon my ignorance but why can't a primary key be NULL ? Is it a rule
>or is it specific to just Oracle ?

By definition, a NULL is for missing, unknown, or inapplicable data. A primary key uniquely identifies a row. So a blank could be a key, but a null can't - it is a rule.

>
>rgds,
>mars

-- 
Joel Garry               joelga_at_rossinc.com               Compuserve 70661,1534
These are my opinions, not necessarily those of Ross Systems, Inc.   <> <>
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