前言：
此刻朋友们对“mysql5156”可能比较关注，兄弟们都想要分析一些“mysql5156”的相关资讯。那么小编也在网络上搜集了一些有关“mysql5156””的相关文章，希望姐妹们能喜欢，小伙伴们快快来了解一下吧！目录说明定义时注意事项三者精度区别float场景的问题是精读问题，需运算、比较的的场景，我们尽量不要用floatfloat(M,D)待遇不带，区别很大,如果不指定D，还会给我们截断某些位总结
说明
本文主要介绍float、double、Decimal的相关知识，以及大家日常使用中的误区，以避免后续日常工作中出现意想不到的bug.定义时注意事项
float(M,D) unsigned 中的M代表可以使用的数字位数，D则代表小数点后的小数位数, unsigned 代表不允许使用负数!double(M,D) unsigned 中的M代表可以使用的数字位数，D则代表小数点后的小数位数decimal(M,D)，M代表可以使用的数字位数，D则代表小数点后的小数位数其中，M 的范围是165，D 的范围是030，而且D不能大于M。三者精度区别
类型名称
存储需求
范围
float
4字节
(-3.402 823 466 E+38，-1.175 494 351 E-38)，0，(1.175 494 351 E-38，3.402 823 466 351 E+38)
double
8字节
(-1.797 693 134 862 315 7 E+308，-2.225073858507 2014E-308)，0，(2.225 073 858 507 201 4 E-308，1.797 693 134 862 315 7 E+308)
decimail(M,D)
具体字节数需要根据实际情况推算
依赖于M和D的值
float场景的问题是精读问题，需运算、比较的的场景，我们尽量不要用float
(root@localhost) [jedmonitor] >create table test_1(id int ,c_float float(24,6) ,c_doubie double(24,6),c_decimal decimal (24,6));Query OK, 0 rows affected (0.00 sec)(root@localhost) [jedmonitor] >insert into test_1 values(1,61.61,61.61,61.61);Query OK, 1 row affected (0.00 sec)(root@localhost) [jedmonitor] >select * from test_1;+------+-----------+-----------+-----------+| id | c_float | c_doubie | c_decimal |+------+-----------+-----------+-----------+| 1 | 61.610001 | 61.610000 | 61.610000 |+------+-----------+-----------+-----------+1 row in set (0.00 sec)--运算结果(root@localhost) [jedmonitor] >select c_float*100000000,c_doubie*100000000,c_decimal*100000000 from test_1;+-------------------+--------------------+---------------------+| c_float*100000000 | c_doubie*100000000 | c_decimal*100000000 |+-------------------+--------------------+---------------------+| 6161000061.035156 | 6161000000.000000 | 6161000000.000000 |+-------------------+--------------------+---------------------+(root@localhost) [jedmonitor] >select * From test_1;+------+-----------+-----------+-----------+| id | c_float | c_doubie | c_decimal |+------+-----------+-----------+-----------+| 1 | 61.610001 | 61.610000 | 61.610000 |+------+-----------+-----------+-----------+1 row in set (0.00 sec)-- 比较(root@localhost) [jedmonitor] >select * from test_1 where c_float=61.61;Empty set (0.00 sec)(root@localhost) [jedmonitor] >select * from test_1 where c_doubie=61.61;+------+-----------+-----------+-----------+| id | c_float | c_doubie | c_decimal |+------+-----------+-----------+-----------+| 1 | 61.610001 | 61.610000 | 61.610000 |+------+-----------+-----------+-----------+1 row in set (0.00 sec)(root@localhost) [jedmonitor] >select * from test_1 where c_decimal=61.61;+------+-----------+-----------+-----------+| id | c_float | c_doubie | c_decimal |+------+-----------+-----------+-----------+| 1 | 61.610001 | 61.610000 | 61.610000 |+------+-----------+-----------+-----------+1 row in set (0.00 sec)float(M,D)待遇不带，区别很大,如果不指定D，还会给我们截断某些位
(root@localhost) [jedmonitor] >create table test_1(id int ,c_float1 float,c_float2 float(12,6));
Query OK, 0 rows affected (0.00 sec)
(root@localhost) [jedmonitor] >insert into test_1 values (1,61.60001,61.60001);
Query OK, 1 row affected (0.00 sec)
(root@localhost) [jedmonitor] >select * from test_1;
±-----±---------±----------+
| id | c_float1 | c_float2 |
±-----±---------±----------+
| 1 | 61.6 | 61.600010 |
±-----±---------±----------+
1 row in set (0.00 sec)
(root@localhost) [jedmonitor] >insert into test_1 values (1,61.6001,61.6001);
Query OK, 1 row affected (0.00 sec)
(root@localhost) [jedmonitor] >insert into test_1 values (1,61.601,61.601);
Query OK, 1 row affected (0.00 sec)
(root@localhost) [jedmonitor] >insert into test_1 values (1,61.61,61.61);
Query OK, 1 row affected (0.00 sec)
(root@localhost) [jedmonitor] >select * from test_1;
±-----±---------±----------+
| id | c_float1 | c_float2 |
±-----±---------±----------+
| 1 | 61.6 | 61.600010 |
| 1 | 61.6001 | 61.600101 |
| 1 | 61.601 | 61.601002 |
| 1 | 61.61 | 61.610001 |
±-----±---------±----------+
总结
1.涉及运算比较，尽量用c_decimal2.如果用了float、double,需要注意制定(m，n),否则可能会出现意外的结果
标签： #mysql5156
