Hi, Too much work and too big discussion change my first clear view on it ;-) One coffee and i am back :) False or undef is undef not undef is still undef
Than all is ok Thanks again and for your patience Regards, Karol Bieniaszewski ----- Reply message ----- Od: "Kjell Rilbe" <[email protected]> Do: "[email protected]" <[email protected]> Temat: Odp: [firebird-support] Bug? Come back Data: śr., maj 7, 2014 10:26 Kjell Rilbe skriver: > > [email protected] skriver: > > > > Hi, > > > > I back with this question. Is this really correct because after > > discussion with team i have now doubt. > > > > We have here parenthesis. Any expression in parenthesis should be > > first calculated > > Then this is not test > > NOT NULL - > NULL only > > Not ( NULL -> NULL) > > > > What do you mean with that last line "Not (NULL -> NULL)"? > > Think of "NULL" as "UNKNOWN" and add that if the value is forced to > true/false NULL evaluates to false, the logic becomes rather intuitive: > > 1. "if NULL then 1 else 2 end" returns 2 > > 2. "if not NULL then 1 else 2 end" returns 2 because "not NULL" is NULL > and the expression degenerates to the same as example 1 above. > > Regards, > Kjell > Sorry, that should be "case when..." not "if...". Same goes for where clauses. Also, note that boolean operators and and or always return NULL if either operand is NULL and that operand has any significance in the expression, so it's not until the entire expression is evaluated and a yes/no decision HAS to be made, that NULL degenerates to false: where NULL and true will not return any records, because "NULL and true" evaluates to "NULL" which then degenerates to false. But where NULL or true will return all records, because the expression will be true no matter what the unknown value would be, so "NULL or true" evaluates to true. Regards, Kjell
