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https://issues.apache.org/jira/browse/DERBY-2212?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Mike Matrigali updated DERBY-2212:
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No, I don't think you have overlooked a compatibility issue. But you do
recogonize there
is a difference in behavior. I have found that no matter how slight the
behavior there will
be some number of applications that depended on it. I would rather see derby
not
introduce this incompatibility.
If the proposal is to introduce the incompatibility I would like to see how
this will be handled in soft vs. hard upgrade and in both cases how it will be
handled by existing indexes vs. indexes created after the soft or hard upgrade.
So I would prefer constraint on null has the new behavior. unique index (which
is not the
same thing as unique constraint) has old behavior. I also don't think we
should create
a new create index with extra syntax for new behavior as the functionality is
fully available
via constraint syntax, but if someone feels it is necessary or more user
friendly I would not
care much.
> 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
>
>
> 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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