Using update statements in create function statements

CREATE OR REPLACE FUNCTION orderupdate(INTEGER, INTEGER) RETURNS INTEGER AS '

UPDATE HEXORDERS

SET ORDER_AMOUNT = (

SELECT SUM(CUSTITEM_QUANTITY * (ITEM_BASEPRICE + ((CUSTITEM_USERS - 1) * ITEM_USEPRICE)))

FROM HEXCUSTITEMS, HEXITEMS

WHERE HEXCUSTITEMS.CUSTOMER_ID = $2

AND HEXCUSTITEMS.ORDER_ID = $1

AND HEXCUSTITEMS.ITEM_ID = HEXITEMS.ITEM_ID

GROUP BY HEXCUSTITEMS.ORDER_ID, HEXCUSTITEMS.CUSTOMER_ID ),

ORDER_GST = (

SELECT SUM((CUSTITEM_QUANTITY * (ITEM_BASEPRICE + ((CUSTITEM_USERS - 1) * ITEM_USEPRICE))) * .1)

FROM HEXCUSTITEMS, HEXITEMS

WHERE HEXCUSTITEMS.CUSTOMER_ID = $2

AND HEXCUSTITEMS.ORDER_ID = $1

AND HEXCUSTITEMS.ITEM_ID = HEXITEMS.ITEM_ID

AND CUSTITEM_GST = TRUE

GROUP BY HEXCUSTITEMS.ORDER_ID, HEXCUSTITEMS.CUSTOMER_ID )

WHERE ORDER_ID = $1

AND CUSTOMER_ID = $2 ;

SELECT 1 ;

' LANGUAGE SQL ;

To be used by:

CREATE TRIGGER HEXCUSTITEMS_TRIGGER1

AFTER INSERT ON HEXCUSTITEMS

FOR EACH STATEMENT

EXECUTE orderupdate( HEXCUSTITEMS.ORDER_ID, HEXCUSTITEMS.CUSTOMER_ID ) ;