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RETURNING SEQUENCE

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Derby Reference Manual

148

The XMLQUERY operator has two arguments, an XML query expression and a Derby
XML value.

Syntax

XMLQUERY ( xquery-string-literal
PASSING BY REF xml-value-expression
[ RETURNING SEQUENCE [ BY REF ] ]
EMPTY ON EMPTY
)

xquery-string-literal

Must be specified as a string literal. If this argument is specified as a parameter,
an expression that is not a literal, or a literal that is not a string (for example an
integer),Derby throws an error. The

xquery-string-literal

argument must

also be an XPath expression that is supported by Apache Xalan. Derby uses Apache
Xalan to evaluate all XML query expressions. Because Xalan does not support full
XQuery, neither does Derby. If Xalan is unable to compile or execute the query
argument, Derby catches the error that is thrown by Xalan and throws the error as a
SQLException. For more on XPath and XQuery expressions, see these Web sites:

http://www.w3.org/TR/xpath

and

http://www.w3.org/TR/xquery/

xml-value-expression

Must be an XML data value and must constitute a well-formed SQL/XML document.
The

xml-value-expression

argument cannot be a parameter. Derby does not

perform implicit parsing nor casting of XML values, so use of strings or any other data
type results in an error. If the argument is a sequence that is returned by a Derby
XMLQUERY operation, the argument is accepted if it is a sequence of exactly one
node that is a document node. Otherwise Derby throws an error.

BY REF

Optional keywords that describe the only value passing mechanism supported by
Derby. Since BY REF is also the default passing mechanism, the XMLQUERY
operator behaves the same whether the keywords are present or not. For more
information on passing mechanisms, see the SQL/XML specification.

RETURNING SEQUENCE

Optional keywords that describe the only XML type returned by the Derby
XMLQUERY operator. Since SEQUENCE is also the default return type, the
XMLQUERY operator behaves the same whether the keywords are present or
not. For more information on the different XML return types, see the SQL/XML
specification.

EMPTY ON EMPTY

Required keywords that describe the way in which XMLQUERY handles an empty
result sequence. The XMLQUERY operator returns an empty sequence exactly as
the sequence is. The XMLQUERY operator does not convert the empty sequence
to a null value. When an empty result sequence is serialized, the result is an empty
string. Derby does not consider an empty result sequence to be a well-formed XML
document.

The result of the XMLQUERY operator is a value of type XML. The result represents a
sequence of XML nodes or values. Atomic values, such as strings, can be part of the
result sequence. The result of an XMLQUERY operator is not guaranteed to represent
a well-formed XML document and it might not be possible to insert the result of an
XMLQUERY operator into an XML column. To store the result in an XML column, the
result must be a sequence with exactly one item in the sequence and the item must be a
well-formed document node. The result can be viewed only in serialized form by explicitly
using the

XMLSERIALIZE operator

Examples
In the

x_table

table, to search the XML column

xcol

and return the students that have

an age attribute that is greater than 20, use the following statement: