Chapter 1: SQL Building Blocks

and and or

and connects two expressions and returns results when both are true. or connects two or more conditions and returns results when either of the conditions is true.

When more than one logical operator is used in a statement, and is evaluated before or. Use parentheses to change the order of execution.

Table 1-7 shows the results of logical operations, including those that involve null values:

**Table 1-7: Truth tables for logical expressions**
and	TRUE	FALSE	NULL
TRUE	TRUE	FALSE	UNKNOWN
FALSE	FALSE	FALSE	FALSE
NULL	UNKNOWN	FALSE	UNKNOWN
or	TRUE	FALSE	NULL
TRUE	TRUE	TRUE	TRUE
FALSE	TRUE	FALSE	UNKNOWN
NULL	TRUE	UNKNOWN	UNKNOWN
not
TRUE	FALSE
FALSE	TRUE
NULL	UNKNOWN

The result UNKNOWN indicates that one or more of the expressions evaluates to NULL, and that the result of the operation cannot be determined to be either TRUE or FALSE.

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