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https://issues.apache.org/jira/browse/DERBY-2212?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12512383
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Oleksandr Alesinskyy commented on DERBY-2212:
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Oracle behavior:
(null,null,null) vs (null,null,null) - allowed
(1,null,null) vs (null,null,null) - allowed
(1,null,null) vs (1,null,null) - NOT allowed
(1,2,null) vs ((1,null,null)) - allowed
(1,2,null) vs (1,2,null) - NOT allowed
(1,2,3) vs (1,2,null) - allowed
(1,2,3) vs (1,2,3l) - NOT allowed
DB2 for iSeries
without "when not null" clause:
(null,null,null) vs (null,null,null) - NOT allowed
(1,null,null) vs (null,null,null) - allowed
(1,null,null) vs (1,null,null) - NOT allowed
(1,2,null) vs ((1,null,null)) - allowed
(1,2,null) vs (1,2,null) - NOT allowed
(1,2,3) vs (1,2,null) - allowed
(1,2,3) vs (1,2,3) - NOT allowed
with "when not null" clause:
(null,null,null) vs (null,null,null) - allowed
(1,null,null) vs (null,null,null) - allowed
(1,null,null) vs (1,null,null) -allowed
(1,2,null) vs ((1,null,null)) - allowed
(1,2,null) vs (1,2,null) - allowed
(1,2,3) vs (1,2,null) - allowed
(1,2,3) vs (1,2,3l) - NOT allowed
> 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
>
> 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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