Dacă te interesează o soluţie pentru SQL 2000, atunci poţi să faci ceva de genul:
SELECT Produs,
  SUM(CASE WHEN DAY(Data)=1 THEN 1 ELSE 0 END) AS [1],
  SUM(CASE WHEN DAY(Data)=2 THEN 1 ELSE 0 END) AS [2],
  SUM(CASE WHEN DAY(Data)=3 THEN 1 ELSE 0 END) AS [3],
  SUM(CASE WHEN DAY(Data)=4 THEN 1 ELSE 0 END) AS [4],
  SUM(CASE WHEN DAY(Data)=5 THEN 1 ELSE 0 END) AS [5],
  SUM(CASE WHEN DAY(Data)=6 THEN 1 ELSE 0 END) AS ,
  SUM(CASE WHEN DAY(Data)=7 THEN 1 ELSE 0 END) AS [7],
  SUM(CASE WHEN DAY(Data)=8 THEN 1 ELSE 0 END) AS ,
  SUM(CASE WHEN DAY(Data)=9 THEN 1 ELSE 0 END) AS [9],
  SUM(CASE WHEN DAY(Data)=10 THEN 1 ELSE 0 END) AS [10],
  SUM(CASE WHEN DAY(Data)=11 THEN 1 ELSE 0 END) AS [11],
  SUM(CASE WHEN DAY(Data)=12 THEN 1 ELSE 0 END) AS [12],
  SUM(CASE WHEN DAY(Data)=13 THEN 1 ELSE 0 END) AS [13],
  SUM(CASE WHEN DAY(Data)=14 THEN 1 ELSE 0 END) AS [14],
  SUM(CASE WHEN DAY(Data)=15 THEN 1 ELSE 0 END) AS [15],
  SUM(CASE WHEN DAY(Data)=16 THEN 1 ELSE 0 END) AS [16],
  SUM(CASE WHEN DAY(Data)=17 THEN 1 ELSE 0 END) AS [17],
  SUM(CASE WHEN DAY(Data)=18 THEN 1 ELSE 0 END) AS [18],
  SUM(CASE WHEN DAY(Data)=19 THEN 1 ELSE 0 END) AS [19],
  SUM(CASE WHEN DAY(Data)=20 THEN 1 ELSE 0 END) AS [20],
  SUM(CASE WHEN DAY(Data)=21 THEN 1 ELSE 0 END) AS [21],
  SUM(CASE WHEN DAY(Data)=22 THEN 1 ELSE 0 END) AS [22],
  SUM(CASE WHEN DAY(Data)=23 THEN 1 ELSE 0 END) AS [23],
  SUM(CASE WHEN DAY(Data)=24 THEN 1 ELSE 0 END) AS [24],
  SUM(CASE WHEN DAY(Data)=25 THEN 1 ELSE 0 END) AS [25],
  SUM(CASE WHEN DAY(Data)=26 THEN 1 ELSE 0 END) AS [26],
  SUM(CASE WHEN DAY(Data)=27 THEN 1 ELSE 0 END) AS [27],
  SUM(CASE WHEN DAY(Data)=28 THEN 1 ELSE 0 END) AS [28],
  SUM(CASE WHEN DAY(Data)=29 THEN 1 ELSE 0 END) AS [29],
  SUM(CASE WHEN DAY(Data)=30 THEN 1 ELSE 0 END) AS [30],
  SUM(CASE WHEN DAY(Data)=31 THEN 1 ELSE 0 END) AS [31],
  COUNT(*) AS Total
FROM Facturi f
INNER JOIN Detail m ON f.ID=m.ID_Factura
INNER JOIN Produse p ON p.ID=m.ID_Produs
WHERE Data BETWEEN '20060801' AND '20060831'
GROUP BY Produs
ORDER BY Produs

Dacă ai SQL Server 2005, atunci poţi să foloseşti PIVOT:

SELECT Produs,
  [1],[2],[3],[4],[5],,[7],,[9],[10],[11],[12],[13],[14],[15],[16],[17],[18],[19],[20],[21],[22],[23],[24],[25],[26],[27],[28],[29],[30],[31],
  [1]+[2]+[3]+[4]+[5]++[7]++[9]+[10]+[11]+[12]+[13]+[14]+[15]+[16]+[17]+[18]+[19]+[20]+[21]+[22]+[23]+[24]+[25]+[26]+[27]+[28]+[29]+[30]+[31] as Total
FROM (
  SELECT Produs, DAY(Data) AS Ziua
  FROM Facturi f
  INNER JOIN Detail m ON f.ID=m.ID_Factura
  INNER JOIN Produse p ON p.ID=m.ID_Produs
  WHERE Data BETWEEN '20060801' AND '20060831'
) AS X PIVOT (COUNT(Ziua) FOR Ziua IN (
  [1],[2],[3],[4],[5],,[7],,[9],[10],[11],[12],[13],[14],[15],[16],[17],[18],[19],[20],[21],[22],[23],[24],[25],[26],[27],[28],[29],[30],[31]
)) AS P
ORDER BY Produs

Răzvan

PS. Evident, semnele alea stupide reprezintă numerele care ar trebui să fie acolo. Nu ştiu cum fac ca să scriu un 6 între paranteze pătrate fără să apară drăcuşorul respectiv...