vb.net - asp.net vb - update database field value automatically when another value is added elsewhere -

i have 2 tables: holidayrequests & holidayentitlement

within holiday requests have 1 field:

• days

when staff member adds holiday request stored number in holiday requests table in days field (there multiple requests different days)

within holidayentitlement table have 3 fields:

• entitlement
• taken
• remaining

when staff member adds holiday request want update both taken , remaining like...

taken = (sum)days staffid = staffid

remaining = entitlement - taken

i've got no idea how go doing this. idea similar of stock management... can point me in right direction can started

you can implement business logic that:

• in database triggers (see other answers)
• in vb code linq2sql using partial methods:

example http://msdn.microsoft.com/en-us/library/bb546176.aspx

partial class customer     private sub onaddresschanged()         ' insert business logic here.     end sub end class