Conditional expression based on if, then,
and else.
The following construct(s) refer to this construct:
An IfExpr implements a conditional expression. The
expression following the if keyword is called the test
expression, the expression following the then keyword is
called the then-expression and the final one is called the
else-expression. If the effective boolean value of the test expression
is true, then the value of the then-expression is returned, otherwise the value
of the else-expression is returned.
In contrast to other languages the else-expression is mandatory. If you do not need the else-expression, you can return an empty string, or a NaN value if you need a numerical value.
Return all books with their titles and net prices:
for $a in input()/bib/book
let $vatrate := 16
let $net := if ($a/price) then xs:decimal($a/price) - xs:decimal($a/price) div ( 100 + $vatrate ) * $vatrate
else xs:double("NaN")
return
<book>
<title>{$a/title}</title>
<price>
<net>{ $net }</net>
</price>
</book>
The conditional expression checks for the presence of a
book child element price
and returns the price minus the value added tax rate. Alternatively, you can
place the IfExpr in the return clause:
for $a in input()/bib/book
let $vatrate := 16
return
<book>
<title>{$a/title}</title>
<price>
<net>{ if ($a/price) then xs:decimal($a/price) - xs:decimal($a/price) div ( 100 + $vatrate ) * $vatrate
else xs:double("NaN") }</net>
</price>
</book>