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unique Constraint [message #8594]

Wed, 03 September 2003 04:21

saravanan
Messages: 70
Registered: October 2000

Member

how many Null values can accept the unique constraint column

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Re: unique Constraint [message #8597 is a reply to message #8594]

Wed, 03 September 2003 05:00

Maaher
Messages: 7065
Registered: December 2001

Senior Member

Unlimited. NULL represents the absence of a value, not a value itself. Oracle can only evaluate values, not the absence of a value, which is why a test like 'X = NULL' will always be 'false' (condition not met).
If you'd try a query like

Select 'Check' test
  From dual
 Where NULL = NULL;

you'd get a 'no rows selected'.

That's the main reason you compare with 'IS NULL' rather than an equasion.
MHE

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