Condition in where clause for default value

Hi

I have a form that takes in a year value and a month value, and feeds it back to a query which returns all transactions for that month in that year.

SELECT People.ID, People.Surname, People.[First Name], VisitHistory.Date, VisitHistory.[Amount collected], Months.Number, Months.Month, Year([Date]) AS [Year]
FROM Months, People INNER JOIN VisitHistory ON People.ID = VisitHistory.[Person ID]
WHERE (((Months.Number)=Month([Date])) AND ((Year([Date]))=[Forms]![Report menu]![Year]) AND ((Month([Date]))=[Forms]![Report menu]![Month]+1));

I want it to display all transactions for a year (i.e. all months) when no month has been selcted on the form?
How do i accompish this.

(Can one put an if statement in the where clause? - such as if no value returned back, select all)

Thanks