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Behavior of Null Values in Logical Expressions

Null values persist through logical expressions in most cases. The following table describes the behavior of null values in logical expressions.

Logical
expression
Result if
x=TRUE
Result if
x=FALSE
Result if
x=.NULL.
x AND .NULL. .NULL. FALSE .NULL.
x OR .NULL. TRUE .NULL. .NULL.
NOT x FALSE TRUE .NULL.

When a conditional expression encounters a null value, the condition fails, because .NULL. is not true (.T.). For example, a FOR clause that evaluates to .NULL. is treated as false (.F.). Note that null values are treated as .NULL. until the entire expression is evaluated.

See Also

FOR Clauses | Null Value Handling | Data Manipulation | Behavior of Null Values in Commands and Functions | NULL as a Value | NULL as a Parameter