Re: Implementation of boolean types.

-CELKO- wrote:
> NULL is not a logical value and UNKNOWN is. Look up the truth tables
> in any SQL book.
>
> Q: Now what is the first and most important rule of NULLs?
> A: NULLs propagate in computations!
>
> That means we should have these rules for Boolean types:
>
> NULL AND TRUE = NULL
> NULL AND FALSE = NULL
> NULL AND NULL = NULL
>
> NULL OR TRUE = NULL
> NULL OR FALSE = NULL
> NULL OR NULL = NULL
>
> NOT NULL= NULL -- valid but really weird looking, unh?

Hmmm...the 3VL truth tables I was taught looked like this (with ? meaning null - I hope you're looking at fixed width fonts):

AND T ? F
  T T ? F
  ? ? ? F
  F F F F

OR T ? F
  T T T T
  ? T ? ?
  F T ? F

  x NOT x
  T F
  ? ?
  F T

This would make your rules for NULL AND FALSE and for NULL OR TRUE incorrect:

   NULL AND FALSE = FALSE

       (because regardless of whether TRUE or FALSE is substituted
        for null, the result will then be false)
   NULL OR TRUE = TRUE
       (because, regardless of whether TRUE or FALSE is substituted
        for null, the overall result will then be TRUE)

> However, in 3VL we have these rules
>
> UNKNOWN AND TRUE = UNKNOWN
> UNKNOWN AND FALSE = FALSE <== opps! not the same as NULL
> UNKNOWN AND UNKNOWN = UNKNOWN
If your initial statement was accurate, then yes; my understanding is that looks the same as the corrected table.

> UNKNOWN OR TRUE = TRUE <== opps! not the same as NULL
> UNKNOWN OR FALSE = UNKNOWN
> UNKNOWN OR UNKNOWN = UNKNOWN
Ditto.

> NOT UNKNOWN = UNKNOWN
>
> So, which of the three values does a NULL map to?

Apparently, NULL == UNKNOWN, under the premises I was taught.

> The results of
> logical expressions now depend on the order of execution and
> substitution of UNNOWN and NULL

Where did I make a mistake in my analysis?

-- 
Jonathan Leffler                   #include <disclaimer.h>
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