Question

I have a mysql 5.1, stored procedure that takes TEXT input which is supposed to contain an XML.

I could successfully parse XML and do everything needed. Now that I am working on error handling part, I need to know how to make sure that the input XML is well formed and valid ?

Thanks

Answer 1

I don’t know of anything in MySQL’s XML support that says “this is well formed XML” but you can fake it. MySQL has an ExtractValue function:

ExtractValue() takes two string arguments, a fragment of XML markup xml_frag and an XPath expression xpath_expr (also known as a locator); it returns the text (CDATA) of the first text node which is a child of the element(s) matched by the XPath expression.

But you can also use this to check the XML since:

If no matching text node is found for the expression (including the implicit /text())—for whatever reason, as long as xpath_expr is valid, and xml_frag consists of elements which are properly nested and closed—an empty string is returned.
[…]
NULL is returned if xml_frag contains elements which are not properly nested or closed, and a warning is generated…

So if you hand ExtractValue any valid XPath expression and any valid piece of XML then you will get a non-NULL result but if you use value XPath and invalid XML, you get a NULL back. So all you need to do is build an INSERT and UPDATE triggers that use ExtractValue to check the XML and signal to raise an exception if you get a NULL:

select ExtractValue(new.your_xml_column, '/*') into @xml_check;
if @xml_check is null then
    signal sqlstate '45000';
end if;