I would like to calculate the minimum value in a group but only if a certain field in that group has a value.
i.e.
Group, InvoiceDate, Earned
001, 1st Jan 2005, £0
001, 1st Mar 2005, £0
001, 1st May 2005, £0
001, 1st Jun 2005, £45.32
001, 1st Jul 2005, £0
002, 1st Jan 2005, £0
002, 1st Feb...