次のようなテーブル構造とサンプルデータがあります。
+----+-----------+------+-------+---------+
| id | item_code | year | month | sales |
+----+-----------+------+-------+---------+
| 1 | 12341 | 2011 | 10 | 1000000 |
+----+-----------+------+-------+---------+
| 2 | 12342 | 2011 | 11 | 2000000 |
+----+-----------+------+-------+---------+
| 3 | 12343 | 2011 | 12 | 3000000 |
+----+-----------+------+-------+---------+
| 4 | 12344 | 2012 | 01 | 4000000 |
+----+-----------+------+-------+---------+
| 5 | 12345 | 2012 | 02 | 5000000 |
+----+-----------+------+-------+---------+
| 6 | 12346 | 2012 | 03 | 6000000 |
+----+-----------+------+-------+---------+
| 7 | 12347 | 2012 | 04 | 6000000 |
+----+-----------+------+-------+---------+
ここで私の質問は、たとえば、現在の月が 2012 年 8 月 (04 - 2012) である場合、次の過去 6 か月のデータをどのように選択するかです。
+----+-----------+------+-------+---------+
| id | item_code | year | month | sales |
+----+-----------+------+-------+---------+
| 1 | 12341 | 2011 | 10 | 1000000 |
+----+-----------+------+-------+---------+
| 2 | 12342 | 2011 | 11 | 2000000 |
+----+-----------+------+-------+---------+
| 3 | 12343 | 2011 | 12 | 3000000 |
+----+-----------+------+-------+---------+
| 4 | 12344 | 2012 | 01 | 4000000 |
+----+-----------+------+-------+---------+
| 5 | 12345 | 2012 | 02 | 5000000 |
+----+-----------+------+-------+---------+
| 6 | 12346 | 2012 | 03 | 6000000 |
+----+-----------+------+-------+---------+
...そして、平均販売額を取得しますか? 何か案は?ティア!!!