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https://issues.apache.org/jira/browse/DERBY-2212?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12539048
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Oleksandr Alesinskyy commented on DERBY-2212:
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New version of specification looks much better, but still has some problems.
1. There are too many typos and they sometimes affect the meaning, e.g.
"null,null,1 null.null,1 no exception" - what is "null.null" (dot instead
of comma)
"null,1,null null,null,1 no exception" - likely should be ""null,1,null
null,1,null no exception"
2. After "no exception" shall be " and table contains both inserted rows"
(otherwise it is legal to assume that second row may be silently swallowed)
3. Issue is named 'Add "Unique where not null" to create index', but discussion
of unique indexes is completely absent from functional specification.
Regards,
Oleksandr
> 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, 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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