In Codd's book, these two Null-type markers are referred to as 'A-Values' and 'I-Values', representing 'Missing But Applicable' and 'Missing But Inapplicable', respectively.
在 Codd的書中，這兩個 Null型標記分別被稱為“A值”和“I值”，分別表示“缺失但適用”和“缺失但不適用”。