15.2. XML Composing Functions in SQL Statements (SQLX)
The preferred means of constructing XML data from SQL is to use the standard SQLX SQL extension.
SQLX is a collection of functions added for creating XML entities from standard relational queries. Basically, you write a SQL statement with calls to SQLX functions in the selection and a piece of XML is created.
There are five XML composing functions:
These functions belong to the SQLX standard, an emerging SQL standard for XML. All the functions take a variable number of arguments.
XMLELEMENT is used to construct
XML elements from relational data. It takes as parameters the
element name, an optional collection of attributes for the element
column names, strings,
XMLCONCAT , and
XMLAGG calls, and an entity objects (returned by
corresponding functions, e.g.
xquery_eval ) which will make up the content of
the element (an exception from this is an attribute entity returned
xquery_eval - in this case it is
joined to the list of the element's attributes). Column names,
strings and attribute entities returned by
xpath_eval will make up the text content of the
element. The others will make up the children of the element.
XMLATTRIBUTES clause, the
value expressions are evaluated to get the values for the
XMLFOREST produces a forest of XML
elements from a given list of arguments. It accepts a list of SQL
value expressions as its arguments, and produces an XML element
from each value returned.
XMLCONCAT produces a forest of
elements by concatenating a list of XML values. XMLCONCAT accepts a
list of XML value expressions as its arguments, and produces a
forest of elements by concatenating the XML values that are
returned from the same row to make one value. If an argument of the
XMLCONCAT is an entity object
path_eval , it must not be an
The following statements create the same result sets:
select XMLELEMENT ('Person', XMLELEMENT ('firstname', "FirstName"), XMLELEMENT ('lastname', "LastName"), xquery_eval('//country', xtree_doc('<a><country>USA</country></a>'))) from "Demo"."demo"."Employees"; select XMLELEMENT ('Person', XMLFOREST ("FirstName"as "firstname", "LastName" as "lastname"), xquery_eval('//country', xtree_doc('<a><country>USA</country></a>'))) from "Demo"."demo"."Employees"; select XMLELEMENT ('Person', XMLCONCAT ( XMLELEMENT ('firstname', "FirstName"), XMLELEMENT ('lastname', "LastName"), xquery_eval('//country', xtree_doc('<a><country>USA</country></a>')))) from "Demo"."demo"."Employees";
The second statement is more effective than the others.
In order to return more than one row of values, you can use
XMLAGG is an aggregate function that produces a
forest of XML elements from a collection of XML elements. It
concatenates the values returned from one column of multiple rows,
unlike XMLCONCAT, which concatenates the values returned from
multiple columns in the same row.
The parameters that would be used as element names (in the
XMLELEMENT and in the 'AS clause ' of
XMLATTRIBUTES ) must be valid XML names. If the
'AS clause ' is absent in a list of the parameters of the
XMLATTRIBUTES , Virtuoso uses the partially
escaped form of the column name as the element or attribute name.
The partially escaped form means that SQL <identifier>
characters that are valid characters in a XML NAME are not changed,
SQL <identifier> character that is not valid XML NAME
character is replaced with "_xHHHH_", where HHHH is character's
upper case hexadecimal code. For example, "first_name" is replaced
with "first_x005F_name", "last name" is replaced with
The following example creates an 'FullAddress' element with
four attributes, three of them ('PostalCode', 'Address', 'City') are produced by
XMLATTRIBUTES, and the fourth attribute - 'country' is calculated by
'Region' subelement, that is produced by
text content, that is produced by
'emp' subelement with text content from the column "LastName", that is created by nested
select XMLELEMENT ('FullAddress', XMLATTRIBUTES ( "PostalCode", "Address", "City"), xtree_doc ('<Region>WA</Region>'), xquery_eval('//@country', xtree_doc('<a country="USA"/>')), xpath_eval('//@Phone', xtree_doc('<a Phone="(206) 555-9857"/>')), XMLELEMENT('emp', "LastName")) from "Demo"."demo"."Employees" ---------------------------- <FullAddress PostalCode="98122" Address="507 - 20th Ave. E. Apt. 2A" City="Seattle" country="USA"> <Region>WA</Region> (206) 555-9857 <emp>Davolio</emp> </FullAddress> <FullAddress PostalCode="98401" Address="908 W. Capital Way" City="Tacoma" country="USA"> <Region>WA</Region> (206) 555-9857 <emp>Fuller</emp> </FullAddress> . . .
XML composing functions deal with arguments of arbitrary type, but the result is always an XML entity that can contain only elements and strings. Hence there is a set of type casting rules. These rules are quite common for any XML DOM model, so they're similar to those listed for DOM function arguments :
If an instance of XMLType is passed then its internal XML entity is used.
If an array representation of an XML tree entity is passed then it is used exactly like XML entity.
If an argument is NULL then it is fully ignored, as if there is no such argument at all.
If an argument is not of a type listed above and not a string then it is cast to a string first.
A root node of some document (or of some generic XML entity) can not appear in the middle of the resulting tree. So if a root node is passed then all child nodes of the root (i.e. every top-level node of the document) will be added.
SQL/XML standards introduce a special name "forest of XML
elements" for an ordered list of XML elements, like one returned by
XMLFOREST() . In Virtuoso, forest can
contain XML nodes of any sort, not only XML elements, so it can
also contain strings, processing instructions and comments.
Virtuoso processes any non-empty "forest" as if it were the root
node of a "generic XML entity", and items of the forest were
top-level nodes of that entity. Hence, a forest can be passed to
any function that accepts an value of type "XML entity". The only
potential problem is that this entity is well-formed if and only if
the forest is non-empty. If an empty forest is serialized to an XML
text then the result is an empty string that is not an acceptable
input for an XML parser.
It is important to remember that the XML document can not contain two neighbour text nodes and that the text node can not be an empty string. If two consequent strings appear in the list of values of a forest or in the list of children of an new element then they are replaced with a single node that is a concatenation of these string. Similarly, if an empty string appears in the list of values of a forest or in the list of children of an new element then it is removed from the list.