不靠谱的FLOAT数据类型
今天在设计数据表时,突然发现原来FLOAT原来是很不靠谱的,所以在这里建议大家换成DOUBLE类型,
原因是:
在mysql手册中讲到,在MySQL中的所有计算都是使用双精度完成的,使用float(单精度)会有误差,出现意想不到的结果。
在我们查询数据时,MySQL使用64位十进制数值的精度执行DECIMAL操作,float(5.54) = 5.54 如果出现精度丢失,这个是不等的。这样,本来我们应该能查到的数据就会莫名其妙的消失。
转自:
https://blog.csdn.net/sunhuaqiang1/article/details/47090367?utm_medium=distribute.pc_relevant.none-task-blog-BlogCommendFromMachineLearnPai2-3.channel_param&depth_1-utm_source=distribute.pc_relevant.none-task-blog-BlogCommendFromMachineLearnPai2-3.channel_param
一、引用刚师兄的一个例子,这是他在他机器上执行的结果
二、我在我电脑上测试如下:
mysql> desc t12; +-------+---------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+---------------+------+-----+---------+-------+ | c1 | float(10,2) | YES | | NULL | | | c3 | decimal(10,2) | YES | | NULL | | +-------+---------------+------+-----+---------+-------+ 2 rows in set (0.00 sec) mysql> desc t13; +-------+---------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+---------------+------+-----+---------+-------+ | c1 | double(10,2) | YES | | NULL | | | c3 | decimal(10,2) | YES | | NULL | | +-------+---------------+------+-----+---------+-------+ 2 rows in set (0.00 sec) mysql> truncate table t12; Query OK, 0 rows affected (0.06 sec) mysql> truncate table t13; Query OK, 0 rows affected (0.05 sec) mysql> insert into t12 values(1234567.23,1234567.23); Query OK, 1 row affected (0.01 sec) mysql> insert into t13 values(1234567.23,1234567.23); Query OK, 1 row affected (0.01 sec) mysql> select * from t12; +------------+------------+ | c1 | c3 | +------------+------------+ | 1234567.25 | 1234567.23 | +------------+------------+ 1 row in set (0.00 sec) mysql> select * from t13; +------------+------------+ | c1 | c3 | +------------+------------+ | 1234567.23 | 1234567.23 | +------------+------------+ 1 row in set (0.00 sec)
三、查询<深入浅出MySQL>原因为: