UNKNOWN is a special value that may be the result of a logical operation, along with the values TRUE and FALSE.
The meaning of the UNKNOWN value is “unknown”, that is, the indefinite result of the operation. The UNKNOWN value can be used when a special NULL value is used in the software development system used. The UNKNOWN value returns a comparison operation if one or both of its operands are NULL, as well as some logical operations if one of their operands is UNKNOWN. By conditional operators of programming languages, the value UNKNOWN is processed similarly to FALSE, that is, a construction of the form
if UNKNOWN then a: = 1 else a: = 2
will cause variable a to be set to 2.
UNKNOWN Transaction Rules
- Any comparison operation of any value with NULL or UNKNOWN results in UNKNOWN.
- not UNKNOWN = UNKNOWN
- TRUE and UNKNOWN = UNKNOWN
- FALSE and UNKNOWN = FALSE
- TRUE or UNKNOWN = TRUE
- FALSE or UNKNOWN = UNKNOWN
- TRUE xor UNKNOWN = UNKNOWN
- FALSE xor UNKNOWN = UNKNOWN
See also
- Trinity Logic
Links
- Hector Garcia-Molina, Jeffrey D. Ullman, Jennifer Weed, “Database Systems. Full course »