XQuery

Logical (and/or) Expressions

Logical expressions combine Boolean values using the operators and and or. They are most often used in conditional (if-then-else) expressions, where clauses of FLWORs and path expression predicates. However, they can be used anywhere a Boolean value is expected.

For example, when used in a conditional expression:

if ($isDiscounted and $discount > 10) then 10 else $discount

an and expression returns true if both of its operands are true. An or expression evaluates to true if one or both of its operands is true.

As with conditional test expressions, the effective Boolean value of each of the operands is evaluated. This means that if the operand expression evaluates to a Boolean false value, the number 0 or NaN, a zero-length string, or the empty sequence, it is considered false; otherwise, it is generally considered true. For example:

$order/item and $numItems

returns true if there is at least one item child of $order, and $numItems (assuming it is numberic) is not equal to 0 or NaN (i.e., not a number).

Evaluation Order of Logical Expressions

The logical operators have lower precedence than comparison operators do, so you can use:

if ($x < 12 and $y > 15) then ...

without parenthesizing the two comparison expressions.

You can also chain multiple and and or expressions together. The and operator takes precedence over the or operator. Therefore:

true( ) and true( ) or false( ) and false( )

is the same as:

(true( ) and true( )) or (false( ) and false( ))

and evaluates ...