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From "Oleksandr Alesinskyy (JIRA)" <j...@apache.org>
Subject [jira] Commented: (DERBY-2212) Add "Unique where not null" to create index
Date Fri, 19 Oct 2007 10:48:50 GMT

    [ https://issues.apache.org/jira/browse/DERBY-2212?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12536178
] 

Oleksandr Alesinskyy commented on DERBY-2212:
---------------------------------------------

Hello Mike,

concerning index and unique constraints - I did not completely catch your point, sorry.
Do you propose to add additional syntax to CONSTRAINT clause only or just go Oracle 
way which treats all unique indexes the same way?

And I completely agree that before implementation some specification concerning both 
SQL syntax and index behavior shall be agreed.

In my opinion, for compatibility reasons DB2 way is better as any index created without 
additional syntax (does not matter in the constraint clause or in the create index command)
behave exactly as before, so no existing application would be affected. And index, created
with additional syntax provides new behavior. BTW, it may be considered as new, third type

of index (that likely require system dictionary expansion).

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
>
>
> 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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