Query filter using a converted CLOB field

Question

Hi,
I need to filter a query by a CLOB field on the database. When I just select the filed, the query runs successfully. But, when I try to filter by this CLOB field, converting it to Date or Char, I got an error message. I detail it step by step below.
Please, can somebody help me to figure out what is happening with this filter?
DB Version: Oracle Database 11g Enterprise Edition Release 11.1.0.7.0
Step 1- The Query works fine
SELECT TO_CHAR( CLOB_FIELD ) FIELD_CLOB_TO_CHAR1,
TO_CHAR(TO_DATE(TO_CHAR( CLOB_FIELD ), 'mm/dd/yyyy', 'NLS_DATE_LANGUAGE=PORTUGUESE'), 'dd/mm/yyyy') FIELD_CLOB_TO_CHAR2,
TO_CHAR(sysdate, 'dd/mm/yyyy', 'NLS_DATE_LANGUAGE=PORTUGUESE') SYSDATE_DATE_TO_CHAR,
TO_DATE(TO_CHAR( CLOB_FIELD ), 'mm/dd/yyyy') FIELD_CLOB_TO_DATE1,
TO_DATE( CLOB_FIELD, 'mm/dd/yyyy') FIELD_CLOB_TO_DATE2
FROM MY_TABLE
FIELD_CLOB_TO_CHAR1: 08/10/2012     
FIELD_CLOB_TO_CHAR2: 10/08/2012
SYSDATE_DATE_TO_CHAR: 23/07/2012
FIELD_CLOB_TO_DATE1: 10/08/2012
FIELD_CLOB_TO_DATE2: 10/08/2012
Step 2- Adding one of the filters below. I got the same error message for them:
and TO_DATE( CLOB_FIELD, 'mm/dd/yyyy') = to_date(to_char(sysdate,'mm/dd/yyyy'), 'mm/dd/yyyy');
and TO_CHAR(TO_DATE(TO_CHAR( CLOB_FIELD ), 'mm/dd/yyyy', 'NLS_DATE_LANGUAGE=PORTUGUESE'), 'dd/mm/yyyy') = to_char(sysdate,'mm/dd/yyyy');
Report Error:
SQL Error: ORA-01858: a non-numeric character was found where a numeric was expected
01858. 00000 - "a non-numeric character was found where a numeric was expected"
*Cause:    The input data to be converted using a date format model was
incorrect. The input data did not contain a number where a number was
required by the format model.
*Action:   Fix the input data or the date format model to make sure the
elements match in number and type. Then retry the operation.
Regards,

Answer 1

user634269 wrote:
Keith, thanks again.
It works fine. I just changed the exception return to show more clearly in which record the problem is. I did:
when others
then
v_returndate := SYSDATE+3000;
But, I still have some doubts and I believe they are related:
1- The dates on CLOB_FIELD are recorded in "mm/dd/yyyy" format. So, when something like 08/09/2011 is found, the things work fine. But, when something like 12/20/2011 is found, I got the problem. I always believed that formatting dates would solve this kind of situation.
2- The function you suggested only converts a varchar parameter into a date value. (simple and clever). I changed the filter from "= sysdate" to "<= sysdate" and got 5 records only. It worked fine. But, considering that the function only applies TO_DATE() on a varchar parameter, why can I do this:
MY_TO_DATE( CLOB_FIELD, 'mm/dd/yyyy') <= to_date(to_char(sysdate,'mm/dd/yyyy'), 'mm/dd/yyyy');
and cannot one of these?
TO_DATE( CLOB_FIELD, 'mm/dd/yyyy') <= to_date(to_char(sysdate,'mm/dd/yyyy'), 'mm/dd/yyyy');
TO_DATE( to_char(CLOB_FIELD), 'mm/dd/yyyy') <= to_date(to_char(sysdate,'mm/dd/yyyy'), 'mm/dd/yyyy')
3- The first function parameter is VARCHAR, but I'm sending the CLOB_FIELD without any conversion. Does Oracle perform an implicit data conversion?
I really would like to understand how Oracle is working with all these things.
Regards.
>Keith, thanks again.
It works fine. I just changed the exception return to show more clearly in which record the problem is. I did:
when others
then
v_returndate := SYSDATE+3000;
But, I still have some doubts and I believe they are related:
1- The dates on CLOB_FIELD are recorded in "mm/dd/yyyy" format. So, when something like 08/09/2011 is found, the things work fine. But, when something like 12/20/2011 is found, I got the problem. I always believed that formatting dates would solve this kind of situation.
You must use the format mask which matches the format of data. Your date data is in the format mm/dd/yyyy and so you need to provide that format mask.
to_date('12/20/2011','mm/dd/yyyy') will cause the data to be correctly converted to a date
2- The function you suggested only converts a varchar parameter into a date value. (simple and clever). I changed the filter from "= sysdate" to "<= sysdate" and got 5 records only. It worked fine. But, considering that the function only applies TO_DATE() on a varchar parameter, why can I do this:
MY_TO_DATE( CLOB_FIELD, 'mm/dd/yyyy') <= to_date(to_char(sysdate,'mm/dd/yyyy'), 'mm/dd/yyyy');
This works because although the implicit conversion fails,  my function suppresses any error that is raised using the following exception block
exception
when others
then
    null;
and cannot one of these?
TO_DATE( CLOB_FIELD, 'mm/dd/yyyy') <= to_date(to_char(sysdate,'mm/dd/yyyy'), 'mm/dd/yyyy');
TO_DATE( to_char(CLOB_FIELD), 'mm/dd/yyyy') <= to_date(to_char(sysdate,'mm/dd/yyyy'), 'mm/dd/yyyy')
These do not work , because you are using the inbuilt functions which are raising errors.
if you change
when others
then
null; to
when others
then
raise; my function will no longer work. You should read the FAQ on exception handling. When others then null is used to suppress any error that is raised. This is not normally good practice.
3- The first function parameter is VARCHAR, but I'm sending the CLOB_FIELD without any conversion. Does Oracle perform an implicit data conversion?
Yes, oracle will try to do an implicit conversion whenever no  explicit conversion is specified.  You should always try to use explicit conversions rather than rely on implicit conversions.
>
I really would like to understand how Oracle is working with all these things.
A good start would be to read and understand the frequently asked questions provided by Blueshadow.
This is also a bad construct:
<= to_date(to_char(sysdate,'mm/dd/yyyy'), 'mm/dd/yyyy')
all you need here is sysdate. If you want to remove the time element, you can just use trunc(sysdate)