Am Donnerstag, den 20.01.2005, 06:09 -0800 schrieb J. Greenlees: > Tino Wildenhain wrote: > > Hi, > > > > Am Mittwoch, den 19.01.2005, 15:02 -0800 schrieb J. Greenlees: > > > >>Roman Neuhauser wrote: > >> > >>># alex@xxxxxxxxxxxxxxx / 2005-01-20 01:35:32 +1100: > >>> > >>> > >>>>i have a unique index on a table over multiple columns. If now one of > >>>>the records has a null value in one of the indexed columns i can insert > >>>>the same record multiple times. > >>>> > >>>>Is this a problem within postgres or expected? > >>> > >>> > >>> In SQL, NULL means "unknown value". How could you assert that two > >>> NULLs are equal? > >>> > >> > >>which doesn't make mathematical sense. > >>mathwise null is an empty result. > >>so setting the logic up using the math logic, null values are always equal. > > > > > > What kind of mathematics you are speaking? > > For example you have "infinity" where infinity is never > > equal to infinity. > > Same with null. Which is "unknown" or "undefined" > > So if x is undefined and y is undefined you cannot > > assume x=y - because if you assume this, then > > they would not be undefined anymore. > > > > q.e.d. > > > > Regards > > Tino > > > > > or null as in empty. > an empty result set is a null set, zero results. > declare a variable, but never assign a value, it has a default value of > null from the declaration. > ( basically any content of memory space allocated that was not actually > empty is the content, but it's a null value to the app. ) No. Empty result set is just a set without elements. zero-length list or tuple or whatever your programming language uses :-) [] != null/undefined/None ---------------------------(end of broadcast)--------------------------- TIP 1: subscribe and unsubscribe commands go to majordomo@xxxxxxxxxxxxxx
