我不太了解cast功能以及decimal此处。decimal(7,2)实际上是什么意思？The first part and the second part arg of the function.

为什么在这里我需要将cast/convert浮点数转换为小数？

mysql> SELECT productCode, MAX(price), MIN(price),
              CAST(AVG(price) AS DECIMAL(7,2)) AS `Average`,
              CAST(STD(price) AS DECIMAL(7,2)) AS `Std Dev`,
              SUM(quantity)
       FROM products
       GROUP BY productCode;
+-------------+------------+------------+---------+---------+---------------+
| productCode | MAX(price) | MIN(price) | Average | Std Dev | SUM(quantity) |
+-------------+------------+------------+---------+---------+---------------+
| PEC         |       0.49 |       0.48 |    0.49 |    0.01 |         18000 |
| PEN         |       1.25 |       1.23 |    1.24 |    0.01 |         15000 |
+-------------+------------+------------+---------+---------+---------------+

下面是相同的sql小提琴吗？

http://sqlfiddle.com/#!2/1ed51b/1/0

我的问题再次重复：

1. decimal(7,2)实际上是什么意思？The first part and the second part arg of the function.
2. 为什么在这里我需要将cast/convert浮点数转换为小数？