Same type of question as:
However, if the start date value is null, then I want my date value to default to the beginning of the month prior.
In my SQL I would write:
IF @StartDate IS NULL
SET @StartDate = Dateadd(mm, Datediff(mm, 0, Dateadd(mm, -1, Getdate())), 0)
How would I write this as an expression for a parameter?