Re: Question abount Unique Keys

Without diving into the docs, the differences I can think of are that a unique index is just that, an index that will not allow duplicate values on the key, as long as none of the columns are nullable.

A unique constraint is a different beastie. It can be referred to by a foreign key, and just as a PK requires an index to help enforce it effeciently, so does a unique constraint.

Jared

On Tuesday 10 April 2001 20:53, Helmut Daiminger wrote:
> Hi!
>
> I have a question about unique keys:
>
> When I issue the statement
> CREATE UNIQUE INDEX UK_TBDOCUMENT ON TBDOCUMENT(DOCUMENTID)
> Oracle creates a unique index (as in user_indexes) but no unique constraint
> (as in user_constraints)
>
> When I issue that statement in a create table statement
> create table....
> CONSTRAINT UK_TBDOCUMENT
> UNIQUE (DOCUMENTID)
> Oracle creates a unique constraint plus an index.
>
> Why is Oracle doing this? Could somebody please shed some light on this?
>
> This is 8.1.6 on Win2k.
>
> Thanks,
> Helmut

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Author: Jared Still
  INET: jkstill_at_cybcon.com

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