Help Needed Urgently ... Pls Pls Pls

Hi,

i have got a database which consists of table, queries, forms and reports.
I have one problem now is that I wan to do a query that will be able to bring the figure from another field to the current one.
ok say i have got Ref_No, Event_Date, Balance_BF, Adjustment, Balance.

so if say this is the first event for this particular ref_no (1234) it will bring the amount from another table (Principle_Table) linking the same ref_no (1234) to the Balance_BF fields,
If this is not the first event for this particular ref_no (1234) it will then bring the amount from the previous event Balance field to the current event Balance_BF field.

I am very lost how to do this. Is there a way doing this using query?

This is wat i mean
Ref No Event Date Balance_BF Adjustment Balance
06/00014 31-May-06 115140 -5,757.00 $109,383.00
06/00014 31-Aug-06 109383 -5,757.00 $103,626.00
06/00014 30-Nov-06 103626 -5,757.00 $97,869.00
06/00014 28-Feb-07 97869 -5,757.00 $92,112.00

U can see that the Balance_BF is actually brought down figure from the previous Balance starting from the second events. For the first event, the Balance_BF is actually draw down from another table (Principles_Table)

Is there a way to do this using query?