On Mon, 16 Oct 2006 19:58:31 -0400, klabu wrote:
> alter table my_emp
> add constraint u_con unique (col_a) ;
>
> create unique index u_index on my_emp (col_a);
>
> What is the difference between Unique Constraint and Unique Index ? And
> under what circumstance(s) will their difference come into play ?
>
>
Constraint is a metadata telling something about the nature of the
column(s). Index is a physical structure which helps when accessing the
table. The difference among the two stems from the difference in their
role (metadata vs. physical access path aid). Any CASE tool or any
sophisticated development tool (project dinosaur by Oracle, froggie by the
Quest software and many others) are likely to read constraints and use
them for creating logical table description. Furthermore, you cannot
create foreign key referencing columns without primary or unique
constraints. At this point I'd have to recommend the RTFM procedure for
obtaining any further details. The FM in this case are the concepts manual
and application developers guide for your database. I would also recommend
not annoying Sybrand any more as he is likely to provide some good
information if he's not cross with you. So, byte the bullet, do some
reading and may the force be with you.
--
http://www.mladen-gogala.com
Received on Mon Oct 16 2006 - 23:00:00 CDT
