创建下表：
mysql> create table t2(id1 float(5,2) default null,id2 double(5,2) default null,id3 decimal(5,2) default null);
mysql> desc t2;
+-------+--------------+------+-----+---------+-------+
| field | type | null | key | default | extra |
+-------+--------------+------+-----+---------+-------+
| id1 | float(5,2) | yes | | null | |
| id2 | double(5,2) | yes | | null | |
| id3 | decimal(5,2) | yes | | null | |
+-------+--------------+------+-----+---------+-------+
往id1,id2,id3这三个字段中插入数据1.23：
mysql> insert into t2(id1,id2,id3) values(1.23,1.23,1.23);
mysql> select * from t2;
+------+------+------+
| id1 | id2 | id3 |
+------+------+------+
| 1.23 | 1.23 | 1.23 |
+------+------+------+
数据都正确插入，再向id1插入1.234，id2插入1.234，id3仍然插入1.23：
mysql> insert into t2(id1,id2,id3) values(1.234,1.234,1.23);
mysql> select * from t2;
+------+------+------+
| id1 | id2 | id3 |
+------+------+------+
| 1.23 | 1.23 | 1.23 |
| 1.23 | 1.23 | 1.23 |
+------+------+------+
数据全部正确插入，但是id1和id2由于标度的限制，舍去了最后一位。
同时向id1,id2,id3中插入数据1.234：
mysql> insert into t2(id1,id2,id3) values(1.234,1.234,1.234);
query ok, 1 row affected, 1 warning (0.02 sec)
mysql> select * from t2;
+------+------+------+
| id1 | id2 | id3 |
+------+------+------+
| 1.23 | 1.23 | 1.23 |
| 1.23 | 1.23 | 1.23 |
| 1.23 | 1.23 | 1.23 |
+------+------+------+
3 rows in set (0.00 sec)
数据也插入成功，但是有一个错误提示，
mysql> show warnings;
+-------+------+------------------------------------------+
| level | code | message |
+-------+------+------------------------------------------+
| note | 1265 | data truncated for column 'id3' at row 1 |
+-------+------+------------------------------------------+
1 row in set (0.00 sec)
将id1,id2,id3的精度和标度都去掉，再插入数据1.234：
mysql> alter table t2 modify id1 float;
query ok, 0 rows affected (0.14 sec)
records: 0 duplicates: 0 warnings: 0
mysql> alter table t2 modify id2 double;
query ok, 0 rows affected (0.04 sec)
records: 0 duplicates: 0 warnings: 0
mysql> alter table t2 modify id3 decimal;
query ok, 4 rows affected, 4 warnings (0.06 sec)
records: 4 duplicates: 0 warnings: 4
mysql> desc t2;
+-------+---------------+------+-----+---------+-------+
| field | type | null | key | default | extra |
+-------+---------------+------+-----+---------+-------+
| id1 | float | yes | | null | |
| id2 | double | yes | | null | |
| id3 | decimal(10,0) | yes | | null | |
+-------+---------------+------+-----+---------+-------+
mysql> insert into t2(id1,id2,id3) values(1.234,1.234,1.234);
query ok, 1 row affected, 1 warning (0.00 sec)
mysql> show warnings;
+-------+------+------------------------------------------+
| level | code | message |
+-------+------+------------------------------------------+
| note | 1265 | data truncated for column 'id3' at row 1 |
+-------+------+------------------------------------------+
1 row in set (0.00 sec)
mysql> select * from t2;
+-------+-------+------+
| id1 | id2 | id3 |
+-------+-------+------+
| 1.234 | 1.234 | 1 |
+-------+-------+------+
1 row in set (0.00 sec)
id1和id2的数据正确插入，而id3被截断。
浮点数如果不写精度和标度，则会按照实际显示，如果有精度和标度，则会将数据四舍五入后插入，系统不报错，定点数如果不设置精度和标度，刚按照默认的（10,0）进行操作，如果数据超过了精度和标度值，则会报错。