mysql> SELECT R_Kunde, COUNT(*) AS "Aufträge" FROM rechnungen WHERE R_Datum BETWEEN "2013-02-01" AND "2013-02-28" GROUP BY R_Kunde;
+---------+-----------+
| R_Kunde | Aufträge |
+---------+-----------+
|       1 |         3 |
|       2 |         1 |
+---------+-----------+

mysql> SELECT R.R_Nr, (RP.Stck*RP.Einzelpreis) AS Summe FROM rechnung_pos AS RP, rechnungen AS R WHERE RP.R_Nr = R.R_Nr AND R.R_Kunde = 1 AND R.R_Datum BETWEEN "2013-02-01" AND "2013-02-28";
+------+-------+
| R_Nr | Summe |
+------+-------+
|    2 |   280 |
|    3 |   100 |
|    4 |   200 |
+------+-------+

mysql> SELECT SUM(RP.Stck*RP.Einzelpreis) AS Summe FROM rechnung_pos AS RP, rechnungen AS R WHERE RP.R_Nr = R.R_Nr AND R.R_Kunde = 1 AND R.R_Datum BETWEEN "2013-02-01" AND "2013-02-28";
+-------+
| Summe |
+-------+
|   580 |
+-------+

mysql> SELECT RP.G_Nr, G.G_Bezeichnung, COUNT(*) AS "Ausgeliehen" FROM rechnung_pos AS RP, geraete as G WHERE RP.G_Nr = G.G_Nr GROUP BY RP.G_Nr ORDER BY 3 DESC;
+------+----------------+-------------+
| G_Nr | G_Bezeichnung  | Ausgeliehen |
+------+----------------+-------------+
|    5 | Thinkpad T430s |           4 |
|    1 | Thinkpad X61   |           1 |
|    3 | Thinkpad T61p  |           1 |
+------+----------------+-------------+