invalid relational error, please help

hi all i am having the following error :

ORA-00920 invalid relational operator

i read the quotes of other people who had the same problem but i didnt understand how this ties to my situation, since it is about memory and relative things, since the formula when working with other tables in my DB shows no problems at all;

the formula is the following :

SELECT TO_CHAR(purinv.Inv_Date,'mm')
MONTH ,TO_CHAR(purinv.Inv_Date,'YYYY') YEAR, SUM(DECODE(PROD.category,'Special',prod.unit_price * sales.qty , 0)) AS "Special",
SUM(DECODE(PROD.category,'Base',prod.unit_price * sales.qty , 0)) AS
"Base",

SUM(DECODE(PROD.category,'Common',prod.unit_price * sales.qty , 0)) AS
"Common"

from prod, sales, purinv
where sales.prod_code = prod.code and Prod.category ('Special', 'Base', 'Common')
and purch.inv_code = purinv.code
group by
TO_CHAR(purinv.Inv_Date,'mm') ,TO_CHAR(purinv.Inv_Date,'YYYY') order by TO_CHAR(purinv.Inv_Date,'YYYY'),TO_CHAR(purinv.Inv_Date,'mm')

any help appreciated Received on Fri Jan 04 2008 - 16:48:39 CST