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https://issues.apache.org/jira/browse/DERBY-2212?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12543859
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Oleksandr Alesinskyy commented on DERBY-2212:
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>functional spec shouldn't have any reference index specially because it talks
>about the feature related to unique constraint.
>How it is implemented shouldn't be mentioned in this document. (Please check
>Mike's feed back on the first version)
I have seen, Mike's feedback, but I do not agree with it. Yes, implementation
details shall be clearly separated from specification itself, but,
if they are important and affect interaction with other parts of the system (as
in this case) they may and shall be mentioned while clearly marked.
I.e. specification structure may be as follows:
Functional specification
1. Specification itself
Bla-bla-bla....
2. Implementation notes
Bla-bla-bla....
> 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
> Assignee: Anurag Shekhar
> Attachments: derby-2212preview.diff, derby-2212preview2.diff,
> FunctionalSpec.html, FunctionalSpecV3.html, FunctionlaSpecv2.html
>
>
> 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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