Finding Monthly averages when some are empty fields

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VB

SQL-Server

So basically i need I have a table that looks almost like this

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Name | Surname | Amount | LastDated | Number of units |
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Duard | Davids | R12000 | 2012/02/05 | 54
Rick | Ford | R5500 | 2012/02/06 | 22
Fey | Morgan | | |

So this should be enough data.

I need to check the last three months data(amounts(adding them up ang getting the average)) and compare it to this months average.

so it would be John : last three months average = R 4500 ,This months average = R 8500.

so the tricky part comes in when getting empty dates(which represent the month the user didnt use the system. in which case the months need to be set back to when the user last used the system and then calculated as an average.

so I started with 4 variables

dim ThisMonth as date = dateTime.now()
dim LastMonth1 as date = thismonth.days.add(-1)
dim LastMonth2 as date = thismonth.days.add(-2)
dim LastMonth3 as date = thismonth.days.add(-3)

so when the month is empty for the LastMonth1 it has to be set back to the closest used month again,but also change the other two accordingly.

any suggestions?