thanks guys for all the help.

I have changed the code to:

use sales in 0 alias sales 
select sales
set order to inv
mkey='003333'
seek mkey 
scan while inv=mkey
    disp PK
endscan
use in sales

and I got all the records.

I misunderstood that

select sales
seek mkey order inv

will automatically set the order, while it's not. I should do:

select sales
set order to inv
seek mkey 

regards,

Jerry