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.