Ms Sql sever 2005

MS sql database table tmp contain following
fields

ModifiedON_A datetime
CreatedOn_A datetime
CreatedBy_A datetime

ModifiedON_B datetime
CreatedOn_B datetime
CreatedBy_B datetime

if the ModifiedON_A date is null then the CreatedOn_A date needs to be checked to see if it meets the timeframe. If CreatedOn_A date is used then the CreatedBy_A value should be used for the ModifiedBy field. If these are all NULL then you use the same process using the ModifiedON_B fields.

thanks plz. reply me soon
in very much hurry !!!