ASA SQL Reference
SQL Language Elements
Search conditions
The syntax for truth-value conditions is as follows:
IS [ NOT ] truth-value
Without the NOT keyword, the condition is TRUE if the condition evaluates to the supplied truth-value, which must be one of TRUE, FALSE, or UNKNOWN. Otherwise, the value is FALSE. The NOT keyword reverses the meaning of the condition, but leaves UNKNOWN unchanged.
Vendor extension. Adaptive Server Enterprise does not support truth-valued conditions.