Create a derived column using SQL to take alternate days

I need a SQL query to insert data in another physical table and a derived column called reporting period should be created based on the FirstCoolOffStartdate. First row of that derived column should be null and second row will always have reporting period value of the respective FirstcoolOff_Created_DT column and also FirstCoolOffEndCycle45Days should show only the 45 days completion date for the alternate days.