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Home -> Community -> Usenet -> c.d.o.server -> Re: Difference between Unique Constraint & Unique Index
A copy of this was sent to "Gilles FLORENTIN" <gflorentin_at_hol.fr>
(if that email address didn't require changing)
On Wed, 2 Sep 1998 10:28:45 +0200, you wrote:
>Hi,
>
>What's the difference between unique constraints and unique indexes.
>
>Thank's
>
a unique constraint will *typically* create a unique index to help it enforce the uniqueness. A unique index is just an index -- its not a constraint.
You can create a foreign key to a set of columns that have a UNIQUE CONSTRAINT on them -- you cannot create a foreign key to a set of columns that simply have a unique index on them.
unique constraints don't always create unique indexes -- if one already exists on that set of column, the unique constraint might 'takeover' that index or if the constraint is defferable -- it won't use a unique index but will use a 'regular' index instead.
Thomas Kyte
tkyte_at_us.oracle.com
Oracle Government
Herndon VA
--
http://govt.us.oracle.com/ -- downloadable utilities
Anti-Anti Spam Msg: if you want an answer emailed to you, you have to make it easy to get email to you. Any bounced email will be treated the same way i treat SPAM-- I delete it. Received on Wed Sep 02 1998 - 10:50:58 CDT
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