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https://issues.apache.org/jira/browse/DERBY-2212?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12658210#action_12658210
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Uwe Kubosch commented on DERBY-2212:
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I have reread the previous comment a dozen times, but I cannot understand what
the solution is.
DERBY-3330 made it into the released 10.4.2.0 version, right?
Could you please give an example of how to create a constraint equaling a
unique index for one or more nullable columns while still allowing multiple
rows with the same values as long as at least one value included in the
constraint in each row is NULL? That is, the constraint should only enforce
uniqueness for rows where _all_ values in the columns included in the
constraint are non-null.
An example would make it clear how to solve the problem. If there is further
documentation on this, please include a reference.
> 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
> Attachments: derby-2212preview.diff, derby-2212preview2.diff,
> FunctionalSpec.html, FunctionalSpecV3.html, FunctionalSpecV3_comment.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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