关于MySQL的小数表示，浮点数与定点数

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浮点数类型：float单精度，double双精度
定点数类型：decimal，默认decimal(10,0)

float占4bytes，double占8bytes，
而decimal的存储要求，整数和小数部分分别计算，每9位数字占用4bytes，而不足9位的，占用情况如下：

需要注意的是，如果小数位不够，采用四舍五入的方法存储，如：

mysql> desc test;
+-------+--------------+------+-----+---------+-------+
| Field | Type         | Null | Key | Default | Extra |
+-------+--------------+------+-----+---------+-------+
| a     | float(3,2)   | YES  |     | NULL    |       |
| b     | double(3,2)  | YES  |     | NULL    |       |
| c     | decimal(3,2) | YES  |     | NULL    |       |
+-------+--------------+------+-----+---------+-------+
3 rows in set (0.00 sec)

mysql> insert into test values (1.236,1.236,1.236);
Query OK, 1 row affected, 1 warning (0.01 sec)

mysql> show warnings;
+-------+------+----------------------------------------+
| Level | Code | Message                                |
+-------+------+----------------------------------------+
| Note  | 1265 | Data truncated for column 'c' at row 1 |
+-------+------+----------------------------------------+
1 row in set (0.00 sec)

mysql> select * from test;
+------+------+------+
| a    | b    | c    |
+------+------+------+
| 1.24 | 1.24 | 1.24 |
+------+------+------+
1 row in set (0.00 sec)