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The [https://en.wikipedia.org/wiki/%3F: ternary if] operator provides a short syntax for conditions. It is also called '''conditional operator''' or '''ternary operator'''. In most languages, the syntax is <code>a ? b : c</code>. As <code>?</code> and <code>:</code> have already been taken in XQuery, the syntax of Perl 6 is used:
<pre classsyntaxhighlight lang="brush:xquery">
$test ?? 'ok' !! 'fails'
</presyntaxhighlight>
The expression returns <code>ok</code> if the effective boolean value of <code>$test</code> is true, and it returns <code>fails</code> otherwise.
The Elvis operator is also available in other languages. It is sometimes called [https://en.wikipedia.org/wiki/Null_coalescing_operator null-coalescing operator]. In XQuery, the value of the first operand will be returned if it is a non-empty sequence. Otherwise, the value of the second operand will be returned.
<pre classsyntaxhighlight lang="brush:xquery">
let $number := 123
return (
$number ?: 0
)
</presyntaxhighlight>
The behavior of the operator is equivalent to the {{Function|Utility|util:or}} function.
In XQuery 3.1, both branches of the <code>if</code> expression need to be specified. In many cases, only one branch is required, so the <code>else</code> branch was made optional in BaseX. If the second branch is omitted, an empty sequence will be returned if the effective boolean value of the test expression is false. Some examples:
<pre classsyntaxhighlight lang="brush:xquery">
if (doc-available($doc)) then doc($doc),
if (file:exists($file)) then file:delete($file),
if (permissions:valid($user)) then <html>Welcome!</html>
</presyntaxhighlight>
If conditions are nested, a trailing else branch will be associated with the innermost <code>if</code>:
<pre classsyntaxhighlight lang="brush:xquery">
if ($a) then if($b) then '$a and $b is true' else 'only $b is true'
</presyntaxhighlight>
In general, if you have multiple or nested if expressions, additional parentheses can improve the readibility of your code:
<pre classsyntaxhighlight lang="brush:xquery">
if ($a) then (
if($b) then '$a and $b is true' else 'only $b is true'
)
</presyntaxhighlight>
The behavior of the if expression is equivalent to the {{Function|Utility|util:if}} function.
'''Example:'''
<pre classsyntaxhighlight lang="brush:xquery">
(: yields "!Hi! !there!" :)
replace('Hi there', '\b', '!', 'j')
</presyntaxhighlight>
=Serialization=
[[Options|Local database options]] can be set in the prolog of an XQuery main module. In the option declaration, options need to be bound to the [[Database Module]] namespace. All values will be reset after the evaluation of a query:
<pre classsyntaxhighlight lang="brush:xquery">
declare option db:chop 'false';
doc('doc.xml')
</presyntaxhighlight>
==XQuery Locks==
If the following XQuery code is called by two clients in parallel, the queries will be evaluated one after another:
<pre classsyntaxhighlight lang="brush:xquery">
declare option basex:write-lock 'CONFIGLOCK';
file:write('config.xml', <config/>)
</presyntaxhighlight>
=Pragmas=
Many optimizations in BaseX will only be performed if an expression is ''deterministic'' (i. e., if it always yields the same output and does not have side effects). By flagging an expression as non-deterministic, optimizations and query rewritings can be suppressed:
<pre classsyntaxhighlight lang="brush:xquery">
sum( (# basex:non-deterministic #) {
1 to 100000000
})
</presyntaxhighlight>
This pragma can be helpful when debugging your code.
In analogy with option declarations and function annotations, [[Transactions#XQuery_Locks|XQuery Locks]] can also set via pragmas:
<pre classsyntaxhighlight lang="brush:xquery">
(# basex:write-lock CONFIGLOCK #) {
file:write('config.xml', <config/>)
}
</presyntaxhighlight>
==Database Pragmas==
* Enforce query to be rewritten for index access. This can e. g. be helpful if the name of a database is not static (see [[Indexes#Enforce Rewritings|Enforce Rewritings]] for more examples):
<pre classsyntaxhighlight lang="brush:xquery">
(# db:enforceindex #) {
for $db in ('persons1', 'persons2', 'persons3')
return db:open($db)//name[text() = 'John']
}
</presyntaxhighlight>
* Temporarily disable node copying in node constructors (see {{Option|COPYNODE}} for more details). The following query will be evaluated faster, and take much less memory, than without pragma, because the database nodes will not be fully copied, but only attached to the new {{Code|xml}} parent element:
<pre classsyntaxhighlight lang="brush:xquery">
file:write(
'wrapped-db-nodes.xml',
}
)
</presyntaxhighlight>
* Specify an XML catalog for URI rewriting. See the [[Catalog Resolver]] section for an example.
'''Query:'''
<pre classsyntaxhighlight lang="brush:xquery">
declare function local:square($a) { $a * $a };
for $i in 1 to 3
return local:square($i)
</presyntaxhighlight>
'''Query after function inlining:'''
<pre classsyntaxhighlight lang="brush:xquery">
for $i in 1 to 3
return
let $a := $i
return $a * $a
</presyntaxhighlight>
'''Query after further optimizations:'''
<pre classsyntaxhighlight lang="brush:xquery">
for $i in 1 to 3
return $i * $i
</presyntaxhighlight>
By default, XQuery functions will be ''inlined'' if the query body is not too large and does not exceed a fixed number of expressions, which can be adjusted via the {{Option|INLINELIMIT}} option.
'''Example:'''
<pre classsyntaxhighlight lang="brush:xquery">
(: disable function inlining; the full stack trace will be shown... :)
declare %basex:inline(0) function local:e() { error() };
local:e()
</presyntaxhighlight>
'''Result:'''
<pre classsyntaxhighlight lang="brush:xml">
Stopped at query.xq, 1/53:
[FOER0000] Halted on error().
Stack Trace:
- query.xq, 2/9
</presyntaxhighlight>
==Lazy Evaluation==
'''Example:'''
<pre classsyntaxhighlight lang="brush:xquery">
declare %basex:lazy variable $january := doc('does-not-exist');
if(month-from-date(current-date()) = 1) then $january else ()
</presyntaxhighlight>
The annotation ensures that an error will only be raised if the condition yields true. Without the annotation, the error will always be raised, because the referenced document is not found.
In analogy with option declarations and pragmas, [[Transactions#XQuery_Locks|XQuery Locks]] can also set via annotations:
<pre classsyntaxhighlight lang="brush:xquery">
declare %basex:write-lock('CONFIGLOCK') function local:write() {
file:write('config.xml', <config/>)
};
</presyntaxhighlight>
=Non-Determinism=
In [http://www.w3.org/TR/xpath-functions-31/#dt-deterministic XQuery], ''deterministic'' functions are “guaranteed to produce ·identical· results from repeated calls within a single ·execution scope· if the explicit and implicit arguments are identical”. In BaseX, many extension functions are non-deterministic or side-effecting. If an expression is internally flagged as non-deterministic, various optimizations that might change their execution order will not be applied.
<pre classsyntaxhighlight lang="brush:xquery">
(: QUERY A... :)
let $n := 456
for $i in 1 to 2
return $n
</presyntaxhighlight>
In some cases, functions may contain non-deterministic code, but the query compiler may not be able to detect this statically. See the following example:
<pre classsyntaxhighlight lang="brush:xquery">
for $read in (file:read-text#1, file:read-binary#1)
let $ignored := non-deterministic $read('input.file')
return ()
</presyntaxhighlight>
Two non-deterministic functions will be bound to <code>$read</code>, and the result of the function call will be bound to <code>$ignored</code>. As the variable is not referenced in the subsequent code, the let clause would usually be discarded by the compiler. In the given query, however, execution will be enforced because of the BaseX-specific {{Code|non-deterministic}} keyword.
In XQuery, some namespaces are statically bound to prefixes. The following query requires no additional namespaces declarations in the query prolog:
<pre classsyntaxhighlight lang="brush:xquery">
<xml:abc xmlns:prefix='uri' local:fn='x'/>,
fn:exists(1)
</presyntaxhighlight>
In BaseX, various other namespaces are predefined. Apart from the namespaces that are listed on the [[Module Library]] page, the following namespaces are statically bound:
* Main modules have an expression as query body. Here is a minimum example:
<pre classsyntaxhighlight lang="brush:xquery">
'Hello World!'
</presyntaxhighlight>
* Library modules start with a module namespace declaration and have no query body:
<pre classsyntaxhighlight lang="brush:xquery">
module namespace hello = 'http://basex.org/examples/hello';
'Hello World!'
};
</presyntaxhighlight>
We recommend {{Code|.xq}} as suffix for for main modules, and {{Code|.xqm}} for library modules. However, the actual module type will dynamically be detected when a file is opened and parsed.
