## 9.14. XML Functions

The functions and function-like expressions described in this section operate on values of type `xml`. Check Section 8.13, “XML Type” for information about the `xml` type. The function-like expressions `xmlparse` and `xmlserialize` for converting to and from type `xml` are not repeated here. Use of most of these functions requires the installation to have been built with `configure --with-libxml`.

### 9.14.1. Producing XML Content

A set of functions and function-like expressions are available for producing XML content from SQL data. As such, they are particularly suitable for formatting query results into XML documents for processing in client applications.

#### 9.14.1.1. `xmlcomment`

    xmlcomment(text)

The function `xmlcomment` creates an XML value containing an XML comment with the specified text as content. The text cannot contain “`--`” or end with a “`-`” so that the resulting construct is a valid XML comment. If the argument is null, the result is null.

Example:

    SELECT xmlcomment('hello');

      xmlcomment
    --------------
     <!--hello-->

#### 9.14.1.2. `xmlconcat`

    xmlconcat(xml[, ...])

The function `xmlconcat` concatenates a list of individual XML values to create a single value containing an XML content fragment. Null values are omitted; the result is only null if there are no nonnull arguments.

Example:

    SELECT xmlconcat('<abc/>', '<bar>foo</bar>');

          xmlconcat
    ----------------------
     <abc/><bar>foo</bar>

XML declarations, if present, are combined as follows. If all argument values have the same XML version declaration, that version is used in the result, else no version is used. If all argument values have the standalone declaration value “yes”, then that value is used in the result. If all argument values have a standalone declaration value and at least one is “no”, then that is used in the result. Else the result will have no standalone declaration. If the result is determined to require a standalone declaration but no version declaration, a version declaration with version 1.0 will be used because XML requires an XML declaration to contain a version declaration. Encoding declarations are ignored and removed in all cases.

Example:

    SELECT xmlconcat('<?xml version="1.1"?><foo/>', '<?xml version="1.1" standalone="no"?><bar/>');

                 xmlconcat
    -----------------------------------
     <?xml version="1.1"?><foo/><bar/>

#### 9.14.1.3. `xmlelement`

    xmlelement(name name [, xmlattributes(value [AS attname] [, ... ])] [, content, ...])

The `xmlelement` expression produces an XML element with the given name, attributes, and content.

Examples:

    SELECT xmlelement(name foo);

     xmlelement
    ------------
     <foo/>

    SELECT xmlelement(name foo, xmlattributes('xyz' as bar));

        xmlelement
    ------------------
     <foo bar="xyz"/>

    SELECT xmlelement(name foo, xmlattributes(current_date as bar), 'cont', 'ent');

                 xmlelement
    -------------------------------------
     <foo bar="2007-01-26">content</foo>

Element and attribute names that are not valid XML names are escaped by replacing the offending characters by the sequence `_x`HHHH`_`, where `HHHH` is the character's Unicode codepoint in hexadecimal notation. For example:

    SELECT xmlelement(name "foo$bar", xmlattributes('xyz' as "a&b"));

                xmlelement
    ----------------------------------
     <foo_x0024_bar a_x0026_b="xyz"/>

An explicit attribute name need not be specified if the attribute value is a column reference, in which case the column's name will be used as the attribute name by default. In other cases, the attribute must be given an explicit name. So this example is valid:

    CREATE TABLE test (a xml, b xml);
    SELECT xmlelement(name test, xmlattributes(a, b)) FROM test;

But these are not:

    SELECT xmlelement(name test, xmlattributes('constant'), a, b) FROM test;
    SELECT xmlelement(name test, xmlattributes(func(a, b))) FROM test;

Element content, if specified, will be formatted according to its data type. If the content is itself of type `xml`, complex XML documents can be constructed. For example:

    SELECT xmlelement(name foo, xmlattributes('xyz' as bar),
                                xmlelement(name abc),
                                xmlcomment('test'),
                                xmlelement(name xyz));

                      xmlelement
    ----------------------------------------------
     <foo bar="xyz"><abc/><!--test--><xyz/></foo>

Content of other types will be formatted into valid XML character data. This means in particular that the characters <, >, and & will be converted to entities. Binary data (data type `bytea`) will be represented in base64 or hex encoding, depending on the setting of the configuration parameter xmlbinary. The particular behavior for individual data types is expected to evolve in order to align the SQL and Postgres Pro data types with the XML Schema specification, at which point a more precise description will appear.

#### 9.14.1.4. `xmlforest`

    xmlforest(content [AS name] [, ...])

The `xmlforest` expression produces an XML forest (sequence) of elements using the given names and content.

Examples:

    SELECT xmlforest('abc' AS foo, 123 AS bar);

              xmlforest
    ------------------------------
     <foo>abc</foo><bar>123</bar>

    SELECT xmlforest(table_name, column_name)
    FROM information_schema.columns
    WHERE table_schema = 'pg_catalog';

                                             xmlforest
    -------------------------------------------------------------------------------------------
     <table_name>pg_authid</table_name><column_name>rolname</column_name>
     <table_name>pg_authid</table_name><column_name>rolsuper</column_name>
     ...

As seen in the second example, the element name can be omitted if the content value is a column reference, in which case the column name is used by default. Otherwise, a name must be specified.

Element names that are not valid XML names are escaped as shown for `xmlelement` above. Similarly, content data is escaped to make valid XML content, unless it is already of type `xml`.

Note that XML forests are not valid XML documents if they consist of more than one element, so it might be useful to wrap `xmlforest` expressions in `xmlelement`.

#### 9.14.1.5. `xmlpi`

    xmlpi(name target [, content])

The `xmlpi` expression creates an XML processing instruction. The content, if present, must not contain the character sequence `?>`.

Example:

    SELECT xmlpi(name php, 'echo "hello world";');

                xmlpi
    -----------------------------
     <?php echo "hello world";?>

#### 9.14.1.6. `xmlroot`

    xmlroot(xml, version text | no value [, standalone yes|no|no value])

The `xmlroot` expression alters the properties of the root node of an XML value. If a version is specified, it replaces the value in the root node's version declaration; if a standalone setting is specified, it replaces the value in the root node's standalone declaration.

    SELECT xmlroot(xmlparse(document '<?xml version="1.1"?><content>abc</content>'),
                   version '1.0', standalone yes);

                    xmlroot
    ----------------------------------------
     <?xml version="1.0" standalone="yes"?>
     <content>abc</content>

#### 9.14.1.7. `xmlagg`

    xmlagg(xml)

The function `xmlagg` is, unlike the other functions described here, an aggregate function. It concatenates the input values to the aggregate function call, much like `xmlconcat` does, except that concatenation occurs across rows rather than across expressions in a single row. See Section 9.20, “Aggregate Functions” for additional information about aggregate functions.

Example:

    CREATE TABLE test (y int, x xml);
    INSERT INTO test VALUES (1, '<foo>abc</foo>');
    INSERT INTO test VALUES (2, '<bar/>');
    SELECT xmlagg(x) FROM test;
            xmlagg
    ----------------------
     <foo>abc</foo><bar/>

To determine the order of the concatenation, an `ORDER BY` clause may be added to the aggregate call as described in Section 4.2.7, “Aggregate Expressions”. For example:

    SELECT xmlagg(x ORDER BY y DESC) FROM test;
            xmlagg
    ----------------------
     <bar/><foo>abc</foo>

The following non-standard approach used to be recommended in previous versions, and may still be useful in specific cases:

    SELECT xmlagg(x) FROM (SELECT * FROM test ORDER BY y DESC) AS tab;
            xmlagg
    ----------------------
     <bar/><foo>abc</foo>

### 9.14.2. XML Predicates

The expressions described in this section check properties of `xml` values.

#### 9.14.2.1. `IS DOCUMENT`

    xml IS DOCUMENT

The expression `IS DOCUMENT` returns true if the argument XML value is a proper XML document, false if it is not (that is, it is a content fragment), or null if the argument is null. See Section 8.13, “XML Type” about the difference between documents and content fragments.

#### 9.14.2.2. `XMLEXISTS`

    XMLEXISTS(text PASSING [BY REF] xml [BY REF])

The function `xmlexists` returns true if the XPath expression in the first argument returns any nodes, and false otherwise. (If either argument is null, the result is null.)

Example:

    SELECT xmlexists('//town[text() = ''Toronto'']' PASSING BY REF '<towns><town>Toronto</town><town>Ottawa</town></towns>');

     xmlexists
    ------------
     t
    (1 row)

The `BY REF` clauses have no effect in Postgres Pro, but are allowed for SQL conformance and compatibility with other implementations. Per SQL standard, the first `BY REF` is required, the second is optional. Also note that the SQL standard specifies the `xmlexists` construct to take an XQuery expression as first argument, but Postgres Pro currently only supports XPath, which is a subset of XQuery.

#### 9.14.2.3. `xml_is_well_formed`

    xml_is_well_formed(text)
    xml_is_well_formed_document(text)
    xml_is_well_formed_content(text)

These functions check whether a `text` string is well-formed XML, returning a Boolean result. `xml_is_well_formed_document` checks for a well-formed document, while `xml_is_well_formed_content` checks for well-formed content. `xml_is_well_formed` does the former if the xmloption configuration parameter is set to `DOCUMENT`, or the latter if it is set to `CONTENT`. This means that `xml_is_well_formed` is useful for seeing whether a simple cast to type `xml` will succeed, whereas the other two functions are useful for seeing whether the corresponding variants of `XMLPARSE` will succeed.

Examples:

    SET xmloption TO DOCUMENT;
    SELECT xml_is_well_formed('<>');
     xml_is_well_formed
    --------------------
     f
    (1 row)

    SELECT xml_is_well_formed('<abc/>');
     xml_is_well_formed
    --------------------
     t
    (1 row)

    SET xmloption TO CONTENT;
    SELECT xml_is_well_formed('abc');
     xml_is_well_formed
    --------------------
     t
    (1 row)

    SELECT xml_is_well_formed_document('<pg:foo xmlns:pg="http://postgresql.org/stuff">bar</pg:foo>');
     xml_is_well_formed_document
    -----------------------------
     t
    (1 row)

    SELECT xml_is_well_formed_document('<pg:foo xmlns:pg="http://postgresql.org/stuff">bar</my:foo>');
     xml_is_well_formed_document
    -----------------------------
     f
    (1 row)

The last example shows that the checks include whether namespaces are correctly matched.

### 9.14.3. Processing XML

To process values of data type `xml`, Postgres Pro offers the functions `xpath` and `xpath_exists`, which evaluate XPath 1.0 expressions.

    xpath(xpath, xml [, nsarray])

The function `xpath` evaluates the XPath expression `xpath` (a `text` value) against the XML value `xml`. It returns an array of XML values corresponding to the node set produced by the XPath expression. If the XPath expression returns a scalar value rather than a node set, a single-element array is returned.

The second argument must be a well formed XML document. In particular, it must have a single root node element.

The optional third argument of the function is an array of namespace mappings. This array should be a two-dimensional `text` array with the length of the second axis being equal to 2 (i.e., it should be an array of arrays, each of which consists of exactly 2 elements). The first element of each array entry is the namespace name (alias), the second the namespace URI. It is not required that aliases provided in this array be the same as those being used in the XML document itself (in other words, both in the XML document and in the `xpath` function context, aliases are local).

Example:

    SELECT xpath('/my:a/text()', '<my:a xmlns:my="http://example.com">test</my:a>',
                 ARRAY[ARRAY['my', 'http://example.com']]);

     xpath
    --------
     {test}
    (1 row)

To deal with default (anonymous) namespaces, do something like this:

    SELECT xpath('//mydefns:b/text()', '<a xmlns="http://example.com"><b>test</b></a>',
                 ARRAY[ARRAY['mydefns', 'http://example.com']]);

     xpath
    --------
     {test}
    (1 row)

    xpath_exists(xpath, xml [, nsarray])

The function `xpath_exists` is a specialized form of the `xpath` function. Instead of returning the individual XML values that satisfy the XPath, this function returns a Boolean indicating whether the query was satisfied or not. This function is equivalent to the standard `XMLEXISTS` predicate, except that it also offers support for a namespace mapping argument.

Example:

    SELECT xpath_exists('/my:a/text()', '<my:a xmlns:my="http://example.com">test</my:a>',
                         ARRAY[ARRAY['my', 'http://example.com']]);

     xpath_exists
    --------------
     t
    (1 row)

### 9.14.4. Mapping Tables to XML

The following functions map the contents of relational tables to XML values. They can be thought of as XML export functionality:

    table_to_xml(tbl regclass, nulls boolean, tableforest boolean, targetns text)
    query_to_xml(query text, nulls boolean, tableforest boolean, targetns text)
    cursor_to_xml(cursor refcursor, count int, nulls boolean,
                  tableforest boolean, targetns text)

The return type of each function is `xml`.

`table_to_xml` maps the content of the named table, passed as parameter `tbl`. The `regclass` type accepts strings identifying tables using the usual notation, including optional schema qualifications and double quotes. `query_to_xml` executes the query whose text is passed as parameter `query` and maps the result set. `cursor_to_xml` fetches the indicated number of rows from the cursor specified by the parameter `cursor`. This variant is recommended if large tables have to be mapped, because the result value is built up in memory by each function.

If `tableforest` is false, then the resulting XML document looks like this:

    <tablename>
      <row>
        <columnname1>data</columnname1>
        <columnname2>data</columnname2>
      </row>

      <row>
        ...
      </row>

      ...
    </tablename>

If `tableforest` is true, the result is an XML content fragment that looks like this:

    <tablename>
      <columnname1>data</columnname1>
      <columnname2>data</columnname2>
    </tablename>

    <tablename>
      ...
    </tablename>

    ...

If no table name is available, that is, when mapping a query or a cursor, the string `table` is used in the first format, `row` in the second format.

The choice between these formats is up to the user. The first format is a proper XML document, which will be important in many applications. The second format tends to be more useful in the `cursor_to_xml` function if the result values are to be reassembled into one document later on. The functions for producing XML content discussed above, in particular `xmlelement`, can be used to alter the results to taste.

The data values are mapped in the same way as described for the function `xmlelement` above.

The parameter `nulls` determines whether null values should be included in the output. If true, null values in columns are represented as:

    <columnname xsi:nil="true"/>

where `xsi` is the XML namespace prefix for XML Schema Instance. An appropriate namespace declaration will be added to the result value. If false, columns containing null values are simply omitted from the output.

The parameter `targetns` specifies the desired XML namespace of the result. If no particular namespace is wanted, an empty string should be passed.

The following functions return XML Schema documents describing the mappings performed by the corresponding functions above:

    table_to_xmlschema(tbl regclass, nulls boolean, tableforest boolean, targetns text)
    query_to_xmlschema(query text, nulls boolean, tableforest boolean, targetns text)
    cursor_to_xmlschema(cursor refcursor, nulls boolean, tableforest boolean, targetns text)

It is essential that the same parameters are passed in order to obtain matching XML data mappings and XML Schema documents.

The following functions produce XML data mappings and the corresponding XML Schema in one document (or forest), linked together. They can be useful where self-contained and self-describing results are wanted:

    table_to_xml_and_xmlschema(tbl regclass, nulls boolean, tableforest boolean, targetns text)
    query_to_xml_and_xmlschema(query text, nulls boolean, tableforest boolean, targetns text)

In addition, the following functions are available to produce analogous mappings of entire schemas or the entire current database:

    schema_to_xml(schema name, nulls boolean, tableforest boolean, targetns text)
    schema_to_xmlschema(schema name, nulls boolean, tableforest boolean, targetns text)
    schema_to_xml_and_xmlschema(schema name, nulls boolean, tableforest boolean, targetns text)

    database_to_xml(nulls boolean, tableforest boolean, targetns text)
    database_to_xmlschema(nulls boolean, tableforest boolean, targetns text)
    database_to_xml_and_xmlschema(nulls boolean, tableforest boolean, targetns text)

Note that these potentially produce a lot of data, which needs to be built up in memory. When requesting content mappings of large schemas or databases, it might be worthwhile to consider mapping the tables separately instead, possibly even through a cursor.

The result of a schema content mapping looks like this:

    <schemaname>

    table1-mapping

    table2-mapping

    ...

    </schemaname>

where the format of a table mapping depends on the `tableforest` parameter as explained above.

The result of a database content mapping looks like this:

    <dbname>

    <schema1name>
      ...
    </schema1name>

    <schema2name>
      ...
    </schema2name>

    ...

    </dbname>

where the schema mapping is as above.

As an example of using the output produced by these functions, Figure 9.1, “XSLT Stylesheet for Converting SQL/XML Output to HTML” shows an XSLT stylesheet that converts the output of `table_to_xml_and_xmlschema` to an HTML document containing a tabular rendition of the table data. In a similar manner, the results from these functions can be converted into other XML-based formats.

Figure 9.1. XSLT Stylesheet for Converting SQL/XML Output to HTML

    <?xml version="1.0"?>
    <xsl:stylesheet version="1.0"
        xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
        xmlns:xsd="http://www.w3.org/2001/XMLSchema"
        xmlns="http://www.w3.org/1999/xhtml"
    >

      <xsl:output method="xml"
          doctype-system="http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"
          doctype-public="-//W3C/DTD XHTML 1.0 Strict//EN"
          indent="yes"/>

      <xsl:template match="/*">
        <xsl:variable name="schema" select="//xsd:schema"/>
        <xsl:variable name="tabletypename"
                      select="$schema/xsd:element[@name=name(current())]/@type"/>
        <xsl:variable name="rowtypename"
                      select="$schema/xsd:complexType[@name=$tabletypename]/xsd:sequence/xsd:element[@name='row']/@type"/>

        <html>
          <head>
            <title><xsl:value-of select="name(current())"/></title>
          </head>
          <body>
            <table>
              <tr>
                <xsl:for-each select="$schema/xsd:complexType[@name=$rowtypename]/xsd:sequence/xsd:element/@name">
                  <th><xsl:value-of select="."/></th>
                </xsl:for-each>
              </tr>

              <xsl:for-each select="row">
                <tr>
                  <xsl:for-each select="*">
                    <td><xsl:value-of select="."/></td>
                  </xsl:for-each>
                </tr>
              </xsl:for-each>
            </table>
          </body>
        </html>
      </xsl:template>

    </xsl:stylesheet>