MySQL浮点型和定点型可以用类型名称后加（M，D）来表示，M表示该值的总共长度，D表示小数点后面的长度，M和D又称为精度和标度，如float(7,4)的可显示为-999.9999，MySQL保存值时进行四舍五入，如果插入999.00009，则结果为999.0001。FLOAT和DOUBLE在不指定精度时，默认会按照实际的精度来显示，而DECIMAL在不指定精度时，默认整数为10，小数为0。
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。
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