使用DBMS_LOB时出错

Question

Below is my query:

SELECT dbms_lob.compare(NVL(original_xmldoc,'Null'),NVL(update_xmldoc,'Null'))
FROM xml_files;

When I execute, I received an error:

ORA-06553: PLS-306: wrong number or types of arguments in call to 'COMPARE'
06553. 00000 -  "PLS-%s: %s"

Is there something wrong with my syntax?

regards,

Nelz Ki

Answer 1

If your data types are CLOB then you can do:

SELECT DBMS_LOB.COMPARE(
         NVL( original_xmldoc, Empty_CLOB() ),
         NVL( updated_xmldoc,  Empty_CLOB() )
       )
FROM xml_files;

However, that probably wouldn't generate the error you are seeing so I am assuming that you are using XMLType datatypes:

SELECT DBMS_LOB.COMPARE(
         NVL( original_xmldoc.getClobVal(), Empty_CLOB() ),
         NVL( updated_xmldoc.getClobVal(),  Empty_CLOB() )
       )
FROM xml_files;

(You could use the magic string 'Null' , as per your example, instead of Empty_CLOB() but it seems to me more natural to compare against an empty string rather than a made up value - however, you know your data better than us so it might be appropriate the other way round.)

Answer 2

From your previous questions your columns are XMLType, not CLOB:

create table xml_files(original_xmldoc xmltype, update_xmldoc xmltype);

SELECT dbms_lob.compare(NVL(original_xmldoc,'Null'),NVL(update_xmldoc,'Null'))
FROM xml_files;

ORA-06553: PLS-306: wrong number or types of arguments in call to 'COMPARE'

The dbms_lob.compare() function expects CLOBs (or BLOBs) or something that be implicitly converted to them. XMLTypes cannot. So you need to explicitly convert them:

SELECT dbms_lob.compare(NVL(x.original_xmldoc.getclobval(),'Null'),
  NVL(x.update_xmldoc.getclobval(),'Null'))
FROM xml_files x;

But you're comparing your fixed string value against an XML document and that might get the wrong result, as you're doing linguistic comparison, essentially between the 'N' of that string and the '<' of an actual XML document. As MTO said, it's probably better to use empty_clob() instead of 'Null' - with a quick test they get the opposite results. And NLS settings could potentially change the result of the fixed-string version - bad in itself.

Arguably if you're only looking for the result to be zero (the same) or non-zero (different for any reason) then it doesn't matter, but it's still better for it to be correct. Maybe someone will look at the actual value later and not realise it's wrong.