Prev month Date is exist consider that other wise take before month

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SQL-Server

SQL-Server-2008

I want to check the condition,

1) If prev month details are available then use that condition in where clause
2) If Prev month details are not available , then use before month date, that is also not available then display before month..

EX:

ID       Date
1        2012-04-20
1        2012-08-2
1        2013-02-23

In this example previous month details are available. So, display those details using that condition

ID       Date
1        2012-04-20
1        2012-08-2

In the above ex i don't have prev month details. now , i want to consider Aug2012 details .