GROUP BY On DECODE

Hi,I have two questions, firstly..
I am hoping there is a way to GROUP BY on a concatenated output like the following decode etc..

SELECT
DECODE(PRIM_REASN,'','Reasons not entered',PRIM_REASN)
|| ' '
||SECDY_REASN
|| ' '
|| TERT_REASN) Downtime_Reasons,
EVENT_DUR/60 Minutes

FROM
DOWNTIME_WEB
WHERE
DOWNTIME_WEB.TIMESTAMP>= (SELECT PRODN_SHIFT.START_DATIME
FROM PRODN_SHIFT
WHERE
PRODN_SHIFT.START_DATIME <=sysdate
AND PRODN_SHIFT.END_DATIME > sysdate)
AND
TRIM(DOWNTIME_WEB.STATUS_TEXT) = 'STOPPED'
AND
TRIM(DOWNTIME_WEB.WORK_CTR_NAME) = 'PACK2'
ORDER BY DOWNTIME_WEB.EVENT_DUR

I have tried to put the DECODE etc into the GROUP BY statement but I get an error - " not a valid GROUP BY clause"

Secondly, is there a way of modifying value in field A based on the value in field B

Erroneous example follows - in an effort to explain what I want to do (from above):

DECODE(PRIM_REASN,'',(DECODE(EVENT_DUR,<200,"Low",>200,"High"),PRIM_REASN)
ie:If PRIM_REASN is null I want EVENT_DUR to be tested to determine the new value for PRIM_REASN.
Except that tthe above won't work and I don't know what the right function/argument should be..

This is my first foray into SQL server, I am trying to migrate some access queries to work with web services and hence Xcelsius.


Cheers.