Converting each row in a column to an XML

Question

Hi everyone,
Using Oracle Database 10g Enterprise Edition Release 10.2.0.4.0 - 64bit
I have a table where there is a comments column which is VARCHAR2. That column contains string data in the form of XML tags.
Like for eg. *<tag_name attr_name="10"/>* (sorry cant provide the exact values as I'm not allowed to)
There are many such rows. Now I want to use these strings and operate on them as XML like using XPath to find certain attributes etc.
I tried using the extract function and to use it i need a XMLType object. So I tried to create one using the XMLType() constructor.
But it gives me the error ORA-19032: Expected XML tag , got no content
When I looked it up on the net, I found that this error number doesn't correspond to the error message that I'm getting. The error msg found on the internet was Expected XML tag string got string
I used this query
SELECT XMLType (<column>) FROM <table>;But when I modified it (based on someone's hunch) to
SELECT XMLType (<column>)
  FROM <table>
WHERE ROWNUM <= 1;The query worked fine. Like it can take only one row.
I also tried like this
SELECT XMLType ('<tag_name attr_name="10"/>')
  FROM DUAL;This too worked fine.
So finally this has left me completely confused. Is there a way to convert every row in a column to an XMLType object so that I can use the extract function to gather information about it.
For the time being I have used REGEXes to write the code. But having the flexibility of xml would be better.
Any help would be appreciated.
Regards,
Arijit
Edited by: Arijit Kanrar on May 23, 2013 5:27 AM

Answer 1

Arijit,
The error message is correct. What you found is merely the generic message with string placeholders.
The error message is also pretty self-explanatory : you can't pass an empty string (NULL) to the XMLType constructor.
SQL> select xmltype('') from dual;
ERROR:
ORA-19032: Expected XML tag , got no content
ORA-06512: at "SYS.XMLTYPE", line 310
ORA-06512: at line 1
no rows selectedYou have to either add a WHERE clause to filter out NULL columns, or use a CASE statement to only convert strings that aren't empty :
SELECT XMLType (<column>)
  FROM <table>
WHERE <column> IS NOT NULL ;
SELECT CASE WHEN <column> IS NOT NULL THEN XMLType (<column>) END
FROM ...And do not use EXTRACT if you want to access scalar values, use EXTRACTVALUE instead :
SQL> select extractvalue(xmltype('<tag_name attr_name="10"/>'), '/tag_name/@attr_name') from dual;
EXTRACTVALUE(XMLTYPE('<TAG_NAMEATTR_NAME="10"/>'),'/TAG_NAME/@ATTR_NAME')
10