condition1 logical-operator condition2

NOT condition

expression IS [ NOT ] { truth-value | NULL }

Search conditions can be used to choose a subset of the rows from a table in a FROM clause in a SELECT statement, or in expressions such as an IF or CASE to select specific values. In UltraLite, every condition evaluates as one of three states: TRUE, FALSE, or UNKNOWN. When combined, these states are referred to as three-valued logic. The result of a comparison is UNKNOWN if either value being compared is the NULL value. Search conditions are satisfied only if the result of the condition is TRUE.

AND   The combined condition is TRUE if both conditions are TRUE, FALSE if either condition is FALSE, and UNKNOWN otherwise.

condition1 OR condition2

OR   The combined condition is TRUE if either condition is TRUE, FALSE if both conditions are FALSE, and UNKNOWN otherwise.

NOT   The NOT condition is TRUE if condition is FALSE, FALSE if condition is TRUE, and UNKNOWN if condition is UNKNOWN.

IS   The condition is TRUE if the expression evaluates to the supplied truth-value, which must be one of TRUE, FALSE, or UNKNOWN. Otherwise, the value is FALSE.

• Three-valued logic
• Comparison operators
• Search conditions in UltraLite

The IS NULL condition is satisfied if the column contains a NULL value. If you use the IS NOT NULL operator, the condition is satisfied when the column contains a value that is not NULL. This example shows an IS NULL condition: WHERE paid_date IS NULL.