Sorry I think I didn't make my question clear enough...
I am now working on the enhancement project the structure of the dbf file a kind of poor design, unfortunately I've got to remain the structure of the dbf file
the actually structure of my maintenance.dbf file is like this

ref_no ser_no c_exiry fee
AAA 0002 2/3/02 $10
BBB 0001 8/3/02 $20
CCC 0002 2/4/03 $10
DDD 0003 3/2/02 $20

I write the SQL as below

select maintenance.ser_no, maintenance.ref_no, max(maintenance.c_expiry),; maintenance.fee from maintenance where ser_no in;
(select cser_no from mainboard) and fee>0 and c_expiry
my desired result should be as below

ref_no ser_no c_exiry fee
BBB 0001 18/3/02 $20
DDD 0003 3/2/02 $20

however because the c_expiry date in record CCC has been filter out by the condition
c_expiry
ref_no ser_no c_exiry fee
AAA 0002 2/3/02 $10<-------undesire record
BBB 0001 8/3/02 $20
DDD 0003 3/2/02 $20
how can I solve the problem....thank you for your help..