Compresser les lignes avec des valeurs nulles et des doublons en lignes uniques

Select
   ID,
   AssnDate,
   AssnTxt,
   Max(IsNull(Sally,0)) AS Sally,
   Max(IsNull(Ted, 0)) As Ted,
   Max(IsNull(Bob, 0)) As Bob
From vwGrades
Group By
  ID,
  AssnDate,
  AssnTxt