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 Posted Friday, April 16, 2010 2:28 PM
 SSC Journeyman Group: General Forum Members Last Login: Thursday, September 22, 2011 12:47 PM Points: 93, Visits: 49
 The correct answer to this question is incorrect.The keyword "NULL" represents the special value NULL.In the context of (relational database) boolean logic, there are three possible values: TRUE, FALSE and NULL. In a boolean context, NULL means NULL. The predicate (e.g.) WHERE 1=NULL evaluates to NULL. This is not unknown, it is very specifically known to be the special value NULL. When combined with other boolean expressions with AND and OR operators, the value of NULL is well defined, it's not "unknown".Yes, NULL CAN be used represent the idea "the value for this expression is unknown". But it certainly is not the only meaning. A NULL value can be used to represent other concepts, such as "a value is not applicable" for a particular row, or a value is "not yet assigned", or that some event has not yet occurred. These meanings are distinct from the concept of UNKNOWN. (We can use a NULL to represent something that is known.The assertion that NULL means UNKNOWN is incorrect. That notion is inadequate and incomplete.
Post #905201
 Posted Wednesday, May 05, 2010 11:29 AM
 SSC Rookie Group: General Forum Members Last Login: Wednesday, May 05, 2010 11:26 AM Points: 32, Visits: 18
 Probably one of the weakest q's on QotD. NULL <> NULL doesn't translate to English AT ALL. Nice try Prad.
Post #916310
 Posted Friday, July 23, 2010 3:35 AM
 SSC-Enthusiastic Group: General Forum Members Last Login: Wednesday, September 11, 2013 9:52 PM Points: 125, Visits: 29
 Very good question indeed. NULL does not mean NULL. try running the following statement. This will always return 'NOT Matched' because NULL cannot be compared to NULL as it means nothing or its unknown. SELECT CASE NULL WHEN NULL THEN 'Match' ELSE 'NOT Matched' END
Post #957841
 Posted Friday, July 23, 2010 5:05 AM
 SSCertifiable Group: General Forum Members Last Login: Today @ 4:58 AM Points: 7,945, Visits: 8,366
 Ameya- Ameyask (7/23/2010)Very good question indeed. NULL does not mean NULL.That erroneous view is a fairly natural consequence of treating NULL as a value which exists in all domains instead of as a marker quite separate from the domain of values indicating the absence of any value. This error was originally committed by Codd, but he corrected it later (and by 1986 was advocating two different NULL markers, one for "Inapplicable and therefore absent" and the other for "Applicable but absent", and a 4-valued logic with values T, F, I and A); unfortunately the SQL crew didn't pick up the correction, still less the idea of two different nulls, and the ANSI standard explicitly states that its single NULL is a special value. A value that denotes the absence of any value is a bit of a loopy concept - presumably it denotes the absence of itself, so it would fit well into Charles Dodgson's works of fiction but not at all well into his textbooks on mathematics and logic. Tom'S iomadh doigh a th’ air cu a mharbhadh gun a thachdadh le ìme
Post #957875
 Posted Friday, July 23, 2010 2:49 PM
 SSCertifiable Group: General Forum Members Last Login: Yesterday @ 12:18 PM Points: 5,603, Visits: 7,802
Post #958287
 Posted Saturday, July 24, 2010 2:39 PM
 SSCertifiable Group: General Forum Members Last Login: Today @ 4:58 AM Points: 7,945, Visits: 8,366
Post #958429
 Posted Sunday, July 25, 2010 1:58 AM
 SSCertifiable Group: General Forum Members Last Login: Yesterday @ 12:18 PM Points: 5,603, Visits: 7,802
 Hi Tom,I don't know the 1979 paper (never been able to find it online), but I don't think it matters much. In my opinion, academics tend to argue so much about terminology that they never realise that they actually agree about the underlying concepts.If you want your columns to be atomic, then a "Birthday" column can only hold a valid date that is the birthday of the represented entity - or nothing at all if (for whatever reason), the birthday of that entity is not recorded in the database. The "nothing at all" needs some representation. In a pen and paper table, that would be an empty cell or a dash (to indicate the author did not forget to fill the cell, but left it blank intentionally). In a relational database, the representation for "nothing at all" is called NULL (and the actual bits-and-bytes representation is left to the vendor). Whether you add NULL as a "special value" into each domain, or calll NULL a "marker" that is not part of any domain is just semantics - as long as we agree that the only information carried by NULL is "nothing here", we are in full agreement. Hugo Kornelis, SQL Server MVPVisit my SQL Server blog: http://sqlblog.com/blogs/hugo_kornelis
Post #958466
 Posted Monday, November 01, 2010 3:32 AM
 Ten Centuries Group: General Forum Members Last Login: 2 days ago @ 8:15 AM Points: 1,126, Visits: 415
 My point of view 'does mean' and 'does equal to' aren't the same. NULL mean NULL, but equals to UNKNOWN. Bad question.
Post #1013723

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