SQL Server - Query Each Group of a Group By
SELECT A, B, C,
  CASE
    WHEN 1 = (
               SELECT COUNT(DISTINCT D)
               FROM MyTable tbl2
               WHERE tbl2.A = tbl.A AND tbl2.B = tbl.B and tbl2.C = tbl.C
             )
      THEN (
             SELECT AVG(D)
             FROM MyTable tbl3
             WHERE tbl3.A = tbl.A AND tbl3.B = tbl.B and tbl3.C = tbl.C
           )
    ELSE SUM(D)
FROM MyTable tbl
GROUP BY A, B, C

Table:
| Code | Qty  | ColumnA | ColumnB |
| A    | 100  | 3       | 100     |
| A    | 100  | 4       | 200     |
| A    | 0    | 0       | 300     |
| B    | 0    | 0       | 400     |
| B    |100   | 1       | 500     |

SELECT Code, SUM(ColumnB),
  (SELECT AVG(ColumnA) FROM Table WHERE Code = tbl.Code AND Qty <> 0) AS ColumnA
FROM Table tbl
GROUP BY Code

| Code | ColumnA | ColumnB |
| A    | 3.5     | 600     |
| B    | 1       | 900     |

SELECT Code
     , AVG(CASE WHEN ColumnA<>0 THEN ColumnA ELSE NULL END) AS ColumnA
     , SUM(ColumnB) AS ColumnB
FROM Table tbl
GROUP BY Code