SQL Group function query?

Question

The below sql code gives me ora - Invalid Number error. How do i solve this ? I guess its related to group by function.

SELECT SUM(RAT_CCY_SELL)/COUNT(dat_last_upd) 
FROM  RD_FCY_RATE_HIST  
WHERE cod_ccy ='EUR' 
AND   to_char( pk_ba_global.dat_process,'mm') - to_char(DAT_LAST_UPD,'mm')='1';

Answer 1

I think you should be using the average function AVG here:

SELECT
    AVG(RAT_CCY_SELL) 
FROM
    RD_FCY_RATE_HIST  
WHERE
    cod_ccy = 'EUR' AND
    TO_CHAR(SYSDATE, 'yyyy-mm') = TO_CHAR(DAT_LAST_UPD, 'yyyy-mm')

I assumed here that the column DAT_LAST_UPD contains the date for each record with regard to determining what data corresponds to the current month. If not, then replace that column.

Answer 2

I think the error is because of subtraction of two strings.

Some info from this link are following

Oracle / PLSQL: ORA-01722 Error Message

Cause

 You executed a SQL statement that tried to convert a string to a number, 
but it was unsuccessful.

Resolution

The option(s) to resolve this Oracle error are:

Option #1
----------
Only numeric fields or character fields that contain 
numeric values can be used in arithmetic operations.
Make sure that all expressions evaluate to numbers.

Option #2
-----------
If you are adding or subtracting from dates, 
make sure that you added/substracted a numeric value from the date.