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From "Anurag Shekhar (JIRA)" <j...@apache.org>
Subject [jira] Commented: (DERBY-2212) Add "Unique where not null" to create index
Date Thu, 12 Jul 2007 20:05:04 GMT

    [ https://issues.apache.org/jira/browse/DERBY-2212?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12512238

Anurag Shekhar commented on DERBY-2212:

SQL 2003 doesn't specifically talks about unique index but if we use 
the description of unique predicate, (as Bernt has summarized 
http://tinyurl.com/yvqloj) two nulls are not equal and duplicate keys 
should be allowed as long as at least one part of the key is null. 

The section describing equals operation also hints that two nulls 
are not equal as "=" operator can't be used on nulls.

> Add "Unique where not null" to create index
> -------------------------------------------
>                 Key: DERBY-2212
>                 URL: https://issues.apache.org/jira/browse/DERBY-2212
>             Project: Derby
>          Issue Type: Improvement
>          Components: SQL
>    Affects Versions:
>            Reporter: Oleksandr Alesinskyy
> Derby prohibits creation of unique constraints on nullable colums (as well if only some
columns in the constraint list are nullable) and treat nulls in unique indexes as normal values
(i.e. only one row with null values in indexed columns may be inserted into the table). This
bahavior is very restrictive, does not completely comply with SQL standards (both letter and
intent) as well as with business needs and intending meaning of NULL values (2 null values
are not considered as equal, this comparision shall return NULL, and for selection criteria
boolean null is treated as FALSE).
> This behavior, as far as I can see, is modelled after DB2 (and differs from behavior
of most other major databases, like SyBase, Oracle, etc.).
> But even DB2 provide some means to alleviate these restrictions, namely "UNIQUE WHERE
NOT NULL" clause for CREATE INDEX statement.
> It will be very good if such "UNIQUE WHERE NOT NULL" clause will be introduced in Derby.
> Regards,
> Oleksandr Alesinskyy

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