SQL Query and Exception ORA-01722

Question

Hi,
I have a table called t_lookup which has three columns Lookup_Value, Description and Lookup_Type all of them are varchar2 type. If I execute the below query, its working fine
SELECT *
FROM (SELECT Lookup_Value
*,To_Number(Description) Dd*
FROM t_Lookup
WHERE Lookup_Type = 'CALENDAR') Tab ;
Here is the result
Lookup_Value .............. Dd
January....................... 1
February..................... 2
March......................... 3
April........................... 4
May........................... 5
June.......................... 6
July........................... 7
August....................... 8
September................... 9
October...................... 10
November................... 11
December................... 12
But when i execute
SELECT *
FROM (SELECT Lookup_Value
*,To_Number(Description) Dd*
FROM t_Lookup
WHERE Lookup_Type = 'CALENDAR') Tab
AND Dd = 9;
then it is throwing ORA exception
ORA-01722: invalid number
Can any one help me on this in order to understand the root cause of this problem? Oracle server I am using is "Oracle9i Release 9.2.0.8.0" .
Thanks

Answer 1

I'm assuming that it is intended that certain descriptions will have non-numeric data in them, right?
Assuming that is the case, the success or failure of the query will depend on how the optimizer happens to choose to execute your query. If it happens to run the TO_NUMBER function before it filters out a row with non-numeric data, you'll get an error. If it happens to run the TO_NUMBER after filtering out all the rows with non-numeric data, the query will be successful. Given that query plans are subject to change, particularly when you eventually upgrade, you probably don't want to just find a SQL query that works, you'll need to ensure that either the query plan never changes or that the query is formulated in such a way that it will never fail.
My first thought would be whether you really need to store numeric data in a VARCHAR2 field in the first place. I'm guessing this is some sort of generic lookup table (which cause a raft of issues, this being one). Can you at least store string, date, and number data in separate columns?
Barring that, can you create your own PL/SQL function that converts a string to a number and catches the exception?
CREATE OR REPLACE FUNCTION my_to_number( p_arg IN varchar2 )
  RETURN NUMBER
IS
  l_num NUMBER;
BEGIN
  l_num := to_number( p_arg );
  RETURN l_num;
EXCEPTION
  WHEN others THEN
    RETURN NULL;
END;That will ensure that the query will be successful regardless of the plan. Note that you would probably want to catch just the ORA-01722 error, not OTHERS, in your real code-- this is just a sample.
Barring that, you'll need to do something to enforce stability in the query plan. A stored outline that forces this query to use the query plan that doesn't generate an error.
Justin