fetch multiple records and sum

fetch multiple records and sum

I would like to fetch all records wit the same unique Id number from one table, add the data in a specific field in each record so I get one total for multiple records ... and then put that total in a separate field in another form.  How do I do this.  


example.....

Employee Time Form

PO number |  Reg Hours |  OT Hours 
12345                  8                  4     
12345                  8                  6
54321                  8                  0
54321                  8                  6


I nvoice Form

PO number | Sum Reg Hours.Employee Time Form | Sum OT Hours.Employee Time Form

12345                              16                                                                  10

54321                              16                                                                   6

I want to fetch records from the Employee Time Form based on the PO number I select in the invoice form.  I then want it to sum all the hours in the Reg Hours field in all the records for that PO. and put that total in my invoice form.  Same with OT hours.