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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 Thu, 18 Oct 2007 17:24:50 GMT

    [ https://issues.apache.org/jira/browse/DERBY-2212?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12535982
] 

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

I'm original reporter :) 
DB2-like approach I have suggested for sole reason that Derby is already very DB2-like in
many areas.
If some other approach will be adopted that allows duplicated NULLs in unique indexes, it
is Ok with me.

In this very case it looks that DB2 approach is somewhat more consistent as Oracle one. Namely,
Oracle treats 
NULLs in indexes in a special manner if and only if all columns in the index contain NULL.
I. e. for
create table EXAMPLE (
    x number,
   y  number
);:

create unique index IDX_EXAMPLE on EXAMPLE(x,y);

insert into EXAMPLE values(null,null);
insert into EXAMPLE values(null,null);

would succeed result in 2 records with NULL values, but

insert into EXAMPLE values(1,null);
insert into EXAMPLE values(1,null);

would cause exception on second insert. 
Such behavior is somewhat cumbersome.

On the other hand Oracle treats unique indexes and unique constraints uniformly.


> 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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