Sybase Adaptive Server Anywhere - Page 263

Chapter 6 SQL Language Elements
245
Three-valued logic
The following tables show how the AND, OR, NOT, and IS logical operators
of SQL work in three-valued logic.
AND TRUE FALSE UNKNOWN
TRUE
TRUE FALSE UNKNOWN
FALSE
FALSE FALSE FALSE
UNKNOWN
UNKNOWN FALSE UNKNOWN
OR TRUE FALSE UNKNOWN
TRUE
TRUE TRUE TRUE
FALSE
TRUE FALSE UNKNOWN
UNKNOWN
TRUE UNKNOWN UNKNOWN
TRUE FALSE UNKNOWN
FALSE TRUE UNKNOWN
IS TRUE FALSE UNKNOWN
TRUE
TRUE FALSE FALSE
FALSE
FALSE TRUE FALSE
UNKNOWN
FALSE FALSE TRUE
Explicit selectivity estimates
The Adaptive Server Anywhere optimizer uses statistical information to
decide upon the best strategy for executing each statement. Adaptive Server
Anywhere automatically gathers and updates these statistics. These statistics
are stored permanently in the database in the column statistics registry.
Statistics gathered while processing one statement are available when
searching for efficient ways to execute subsequent statements.
Occasionally, the statistics may become inaccurate or relevant statistics may
be unavailable. This condition is most likely to arise when few queries have
been executed since a large amount of data was added, updated, or deleted.
AND operator
OR operator
NOT operator
IS operator