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https://issues.apache.org/jira/browse/DERBY-2212?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16237007#comment-16237007
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Rick Hillegas commented on DERBY-2212:
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Right. The unique index treats NULL as a distinct, known value. In contrast,
the unique constraint treats NULL as an unknown value. Duplicate violations
occur when known values collide. The good news is that a unique constraint
creates a backing index which the query optimizer uses in exactly the same way
as an ordinary index. It seems to me that the unique constraint will give you
the duplicate checking you want (nulls do not raise duplicate violations)
combined with the query optimizations you need. Is there some reason that your
application will not behave as you want if you declare a unique constraint
rather than a unique index?
> 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: 10.2.1.6
> Reporter: Oleksandr Alesinskyy
> Priority: Major
> Labels: derby_triage10_10
> Attachments: FunctionalSpec.html, FunctionalSpecV3.html,
> FunctionalSpecV3_comment.html, FunctionlaSpecv2.html, derby-2212preview.diff,
> derby-2212preview2.diff
>
>
> 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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