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Null Vrednosti (2)<br />
• Poređenje sa null daje unknown<br />
–Primer: 5 < null ili null null ili null = null<br />
• Tro-vrednosna logika koristi istinosnu vrednost<br />
unknown:<br />
–OR: (unknown or true) = true,<br />
(unknown or false) = unknown<br />
(unknown or unknown) = unknown<br />
– AND: (true and unknown) = unknown,<br />
(false and unknown) = false,<br />
(unknown and unknown) = unknown<br />
– NOT: (not unknown) = unknown<br />
–“P is unknown” je true ako je predikat P unknown<br />
• Rezultat predikata komande where se tretira kao<br />
false ako je unknown