Oracle 11G Clob column with where clause
Prologue

You are not having problems with a "CLOB column with a WHERE clause".
You are having problems processing XML Documents stored in a CLOB field.

This is your third question regarding this task.

I recommend you Read The Fine Manual before you advance on your problem:  Oracle XML Developer's Guide

existsNode

existsNode is deprecated.  Don't use it.

Instead, use:

WHERE XMLExists( '/mt[Error="0"]' passing xml_data)


PL/SQL Error

I suspect this might be due to a conversion error.

You should start investigating this error by fixing your Schema such that you are storing numbers as NUMBER, dates as DATE, and XML Documents as XMLType.  Even if this isn't the case, your Schema would be better for it.

My other thought is: your XML Document has a string but you are attempting to extract a number.