SELECT CASE WHEN RN>1 THEN '' ELSE 單別 END 單別,
CASE WHEN RN>1 THEN '' ELSE 單號 END 單號,
CASE WHEN RN>1 THEN '' ELSE 品號 END 品號,
數量
FROM (
SELECT RN = ROW_NUMBER() OVER(PARTITION BY 單別,單號,品號 ORDER BY  單別),
        單別,
        單號,
        品號,
        數量
FROM TABLE_NAME
) RankTABLE

這不關SQL，只是你在輸出EXCEL(或其他表格)時，不再出跟上一行有相同鍵值內容的部份資料而已
以下假設你的MASTER跟DETAIL的關聯鍵是單號，語言也是我慣用的C#

var worksheet = excel.workbook.worksheets.Add("sheet1");
var Master = (from m in MSSQLDB.MasterTable
             where m.date >= new DateTime(2019,5,1) and m.date <= new DateTime(2019,5,31)
             order by m.單號
             select m).ToArray(); 
var Detail = (from d in MSSQLDB.DetailTable
             where Master.Select(x=>x.單號).Contain(d.單號)
             select d).ToArray(); 
// 以上是假設資料，我習慣一次就把所需資料撈到記憶體裡操作，而不是在資料庫cursor上

int rr = 1 ;// excel橫向列
forech(var md in Master){
    worksheet.cells[rr,1].value = md.單別;
    worksheet.cells[rr,2].value = md.單號;
    foreach(var dd in Detail.Where(p=>p.單號==md.單號){
        worksheet.cells[rr,3].value = dd.品號;
        worksheet.cells[rr,4].value = dd.數量;
        rr++;
    }
    rr++;
}
worksheet.save();//或網頁輸出 Response.BinaryWrite(excel.GetAsByteArray());或你慣用的輸出方式