XML內容生成部分 
SQL數據生成XML的函數。 
1. xmlcomment：生成注釋函數。 
xmlcomment(text ) 
例： 

SELECT xmlcomment('hello'); 
xmlcomment 
-------------- 
<!--hello--> 

2. xmlconcat：XML連接函數 
xmlconcat(xml [, ...]) 
例： 
SELECT xmlconcat('<abc/>', '<bar>foo</bar>'); 

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

連接的XML數據中如果有多個版本聲明的話，連接后的XML只有一個版本聲明。 
例： 
SELECT xmlconcat('<?xml version="1.1"?><foo/>', '<?xml version="1.1" standalone="no"?><bar/>'); 

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

3. xmlelement：生成XML元素函數 
xmlelement(name name [, xmlattributes( value [AS attname ] [, ... ])] [ , content, ... ]) 
例： 
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> 

如果有非法字符的話，非常字符會用 16進制的數字表示出來。 
例子：SELECT xmlelement(name "foo$bar", xmlattributes('xyz' as "a&b")); 
xmlelement 
---------------------------------- 
<foo_x0024_bar a_x0026_b="xyz"/> 

4. xmlforest：生成XML FOREST函數 
xmlforest(content [AS name ] [, ...]) 
例： 
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> 
... 
5. xmlpi：生成XML處理命令函數。 
xmlpi(name target [, content ]) 
例： 
SELECT xmlpi(name php, 'echo "hello world";'); 
xmlpi 
----------------------------- 
<?php echo "hello world";?> 

6. xmlroot：修改XML值的根節點屬性函數 
xmlroot(xml , version text |no value [, standalone yes|no|no value]) 
例子：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> 

7. xmlagg：xmlagg是集約函數 
xmlagg(xml ) 
例： 
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/> 

可以用下面類型的方法改變連接順序。 
SELECT xmlagg(x) FROM (SELECT * FROM test ORDER BY y DESC) AS tab; 
xmlagg 
---------------------- 
<bar/><foo>abc</foo> 

Processing XML 
為了處理XML數據，PostgreSL中提供了xpath函數。 
xpath(xpath , xml [, nsarray ]) 

例： 
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) 

XML和table的映射 
以下函數可以導出XML。 
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) 
這些函數的返回值都是XML類型。 

還有以下函數。具體內容可以參看用戶手冊。 
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) 
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) 
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)