sql help pls

Applications Programming
1

Can anyone help come up with an sql code for the below?

The fields are as follows:
SerialNum
GroupCode
TktType
DueDate
Amount

Examples values are:
1.
SN123456
GC0001
Start
15-OCT-2008
28,500

SN123460
GC0001
End
28-NOV-2008
-28,500

SN134050
GC0002
Start
31-OCT-2008
53,750

SN134065
GC0002
End
30-APR-2009
-53,750

SN210010
GC0004
Start
30-SEP-2008
-18,000

SN210020
GC0004
End
31-OCT-2008
18,000

I need to get the total amount per GroupCode, but only for those outstanding DueDate.
So for the above example rows, only the following will be shown in my output:

SN123460 --> GC001 --> 28500
SN134050 --> GC002 --> 53750
SN134065 --> GC002 --> -53750
SN210020 --> GC004 --> 18000

NOTE: The amounts of the End TktType are always in absolute values if the Start TktType has expired (See output #1)

2 
select GroupCode, sum(Amount)
from mytable
where DueDate < SYSDATE
group by GroupCode;

SYSDATE is today's date in Oracle sql. If using another db, change the syntax.