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