Database Management
Reference Manual
Expressions
: Unset Values
Unset Values
A particular class of undefined values are unset values. The concept exists for attributes which are valid for a given element, but for which no value has been assigned. Typically these may be elements of an array, or ’word’ attributes. References of value =0/0 are also treated as unset.
Unset values are propagated as for undefined values (except for Boolean operations- see below). Undefined values take precedence over unset. There is a specific logical function UNSET to test if a values is unset.
Across comparisons, unset values are not propagated, but are treated as follows:
Operator
When Applied to an UNSET
EQ, GT, GE, LT, LE
Results in FALSE.
NE
Results in TRUE.
OR , AND
Values are treated as FALSE.
For example, if
DESP(2)
and
LVAL(3)
are unset then:
(DESP(2) GT 99) -> False
(DESP(2) NE 33) -> True
(:LVAL(3) AND TRUE) -> False
1974 to current year.
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