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Date: Sat, 4 Aug 2012 17:40:02 +0000 (UTC)
From: "Lukas Eder (JIRA)" <jira@apache.org>
To: derby-dev@db.apache.org
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Subject: [jira] [Commented] (DERBY-5893) Add support for the SQL:2008
 standard IS [ NOT ] DISTINCT FROM predicate
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    [ https://issues.apache.org/jira/browse/DERBY-5893?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13428647#comment-13428647 ] 

Lukas Eder commented on DERBY-5893:
-----------------------------------

Yes, it's different in the way it handles NULLs. For example:

- (1 != NULL) yields NULL, whereas (1 IS DISTINCT FROM NULL) yields TRUE
- (NULL != NULL) yields NULL, whereas (NULL IS DISTINCT FROM NULL) yields FALSE

Here's how you could translate IS [ NOT ] DISTINCT FROM into a equivalent CASE expressions:

{code}
-- A IS DISTINCT FROM B:
CASE WHEN A IS NULL AND B IS NULL THEN 0
     WHEN A IS NULL AND B IS NOT NULL THEN 1
     WHEN A IS NOT NULL AND B IS NULL THEN 1
     WHEN A = B THEN 0
     ELSE 1
END

-- A IS NOT DISTINCT FROM B:
CASE WHEN A IS NULL AND B IS NULL THEN 1
     WHEN A IS NULL AND B IS NOT NULL THEN 0
     WHEN A IS NOT NULL AND B IS NULL THEN 0
     WHEN A = B THEN 1
     ELSE 0
END
{code}

Alternatively, replace 1 and 0 by TRUE and FALSE.
                
> Add support for the SQL:2008 standard IS [ NOT ] DISTINCT FROM predicate
> ------------------------------------------------------------------------
>
>                 Key: DERBY-5893
>                 URL: https://issues.apache.org/jira/browse/DERBY-5893
>             Project: Derby
>          Issue Type: Improvement
>          Components: SQL
>    Affects Versions: 10.9.1.0
>            Reporter: Lukas Eder
>            Priority: Minor
>              Labels: features
>
> The SQL:1999 standard specifies the IS [ NOT ] DISTINCT FROM predicate in chapter 8.15 <distinct predicate>:
> <distinct predicate> ::=
> <row value predicand 3> <distinct predicate part 2>
> <distinct predicate part 2> ::=
> IS [ NOT ] DISTINCT FROM <row value predicand 4>
> <row value predicand 3> ::=
> <row value predicand>
> <row value predicand 4> ::=
> <row value predicand>
> This predicate is supported by at least these databases:
> - http://www.postgresql.org/docs/9.1/static/functions-comparison.html
> - http://www.h2database.com/html/grammar.html#condition_right_hand_side
> - http://hsqldb.org/doc/guide/ch05.html#N11BB0
> - http://dev.mysql.com/doc/refman/5.6/en/comparison-operators.html#operator_equal-to (with a different syntax)
> - http://dcx.sybase.com/1200/en/dbreference/is-distinct-from-search-condition.html
> It would probably make sense for the Derby database to implement it as well.

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