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From:Martijn Tonies Date:April 27 2005 11:39am
Subject:Re: why NOT NULL in PRIMARY key??
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> > [snip]
> > The same is true for any other value... Now that the columns have a NOT
> > NULL
> > constraint the records that previously contained NULL now hold '0'.
> >
> > x y
> > x 0
> > x z
> > x 0
> >
> > Now, how do you uniquely identify the 2nd and 4th rows?
> > [/snip]
> >
> > The database would have thrown an error when you tried to create row 4.
>
> No.  NULL is not a value.  It is a lack of value.  Kind of a special
> symbol, like infinity in mathematics.  You cannot say that

Actually, it's not even the "lack of value".

NULL is a state. A column can have two states: NULL or NON NULL.

Hence:

WHERE mycolumn IS NOT NULL
or
mycolumn IS NULL

Besides a non-null state, it can have many values :-)

With regards,

Martijn Tonies
Database Workbench - developer tool for InterBase, Firebird, MySQL & MS SQL
Server
Upscene Productions
http://www.upscene.com

Thread
why NOT NULL in PRIMARY key??Jigal van Hemert26 Apr
  • Re: why NOT NULL in PRIMARY key??Martijn Tonies26 Apr
  • Re: why NOT NULL in PRIMARY key??Jochem van Dieten26 Apr
  • Re: why NOT NULL in PRIMARY key??Jigal van Hemert26 Apr
  • Re: why NOT NULL in PRIMARY key??Harald Fuchs26 Apr
  • Re: why NOT NULL in PRIMARY key??Jigal van Hemert26 Apr
    • Re: why NOT NULL in PRIMARY key??SGreen26 Apr
      • Att. Shawn GreenJigal van Hemert2 May
  • Re: why NOT NULL in PRIMARY key??Dawid Kuroczko26 Apr
  • Re: why NOT NULL in PRIMARY key??Jigal van Hemert26 Apr
    • Re: why NOT NULL in PRIMARY key??Jochem van Dieten26 Apr
  • Re: why NOT NULL in PRIMARY key??Jigal van Hemert26 Apr
    • Re: why NOT NULL in PRIMARY key??Paul DuBois26 Apr
  • Re: why NOT NULL in PRIMARY key??Martijn Tonies26 Apr
  • Re: why NOT NULL in PRIMARY key??Mikhail Entaltsev26 Apr
  • Re: why NOT NULL in PRIMARY key??Jigal van Hemert26 Apr
  • Re: why NOT NULL in PRIMARY key??Harald Fuchs26 Apr
  • Re: why NOT NULL in PRIMARY key??Martijn Tonies26 Apr
  • Re: why NOT NULL in PRIMARY key??Jigal van Hemert27 Apr
    • Re: why NOT NULL in PRIMARY key??Peter Brawley27 Apr
  • Re: why NOT NULL in PRIMARY key??Jigal van Hemert27 Apr
    • Re: why NOT NULL in PRIMARY key??Frank Bax27 Apr
    • Re: why NOT NULL in PRIMARY key??Peter Brawley27 Apr
    • Re: why NOT NULL in PRIMARY key??Joerg Bruehe27 Apr
  • Re: why NOT NULL in PRIMARY key??Jigal van Hemert28 Apr
RE: why NOT NULL in PRIMARY key??Jay Blanchard26 Apr
  • Re: why NOT NULL in PRIMARY key??Jigal van Hemert26 Apr
    • Re: why NOT NULL in PRIMARY key??Paul DuBois26 Apr
  • RE: why NOT NULL in PRIMARY key??Frank Bax26 Apr
  • Re: why NOT NULL in PRIMARY key??Jigal van Hemert26 Apr
    • Re: why NOT NULL in PRIMARY key??Paul DuBois26 Apr
  • Re: why NOT NULL in PRIMARY key??Martijn Tonies26 Apr
  • Re: why NOT NULL in PRIMARY key??Jigal van Hemert26 Apr
    • Re: why NOT NULL in PRIMARY key??Paul DuBois26 Apr
    • Re: why NOT NULL in PRIMARY key??Peter Brawley26 Apr
RE: why NOT NULL in PRIMARY key??emierzwa26 Apr
  • RE: why NOT NULL in PRIMARY key??SGreen26 Apr
  • Re: why NOT NULL in PRIMARY key??Martijn Tonies26 Apr
RE: why NOT NULL in PRIMARY key??Jay Blanchard26 Apr
  • Re: why NOT NULL in PRIMARY key??Dawid Kuroczko27 Apr
  • Re: why NOT NULL in PRIMARY key??Martijn Tonies27 Apr
RE: why NOT NULL in PRIMARY key??emierzwa26 Apr
Re: why NOT NULL in PRIMARY key??beacker30 Apr
  • Re: why NOT NULL in PRIMARY key??Jigal van Hemert1 May
  • Re: why NOT NULL in PRIMARY key??Martijn Tonies1 May