mysql float double 类型

1.float类型

float列类型默认长度查不到结果，必须指定精度，

比如 num float, insert into table (num) values (0.12); select * from table where num=0.12的话，empty set。

num float(9,7), insert into table (num) values (0.12); select * from table where num=0.12的话会查到这条记录。
mysql> create table tt

-> (

-> num float(9,3)

-> );

Query OK, 0 rows affected (0.03 sec)
mysql> insert into tt(num)values(1234567.8);

ERROR 1264 (22003): Out of range value for column 'num' at row 1

注：超出字段范围，无法插入
mysql> insert into tt(num)values(123456.8);

Query OK, 1 row affected (0.00 sec)
mysql> select
* from tt;

+------------+

| num |

+------------+

| 123456.797 |

+------------+

1 row in set (0.00 sec)

注：小数位数不够，自动补齐，但是存在一个问题就是如上的近似值。
mysql> insert into tt(num)values(123456.867);

Query OK, 1 row affected (0.04 sec)
mysql> select * from
tt;

+------------+

| num |

+------------+

| 123456.797 |

| 123456.797 |

| 123456.867 |

+------------+

3 rows in set (0.00 sec)
mysql> select
* from tt where num=123456.867;

+------------+

| num |

+------------+

| 123456.867 |

+------------+

1 row in set (0.00 sec)
mysql> insert into tt(num)values(2.8);

Query OK, 1 row affected (0.04 sec)
mysql> select * from
tt;

+------------+

| num |

+------------+

| 123456.797 |

| 123456.797 |

| 123456.867 |

| 2.800 |

+------------+

4 rows in set (0.00 sec)
mysql> select
* from tt where num=2.8;

+-------+

| num |

+-------+

| 2.800 |

+-------+

1 row in set (0.00 sec)
mysql> insert into tt(num)values(2.888888);

Query OK, 1 row affected (0.00 sec)
mysql> select
* from tt;

+------------+

| num |

+------------+

| 123456.797 |

| 123456.797 |

| 123456.867 |

| 2.800 |

| 2.889 |

+------------+

5 rows in set (0.00 sec)

注：小数位数超了，自动取近似值。
--------------------------------------------------------------------------------------
2.double类型
mysql> create table tt(

-> num double(9,3)

-> );

Query OK, 0 rows affected (0.02 sec)
mysql> insert into tt(num) values(234563.9);

Query OK, 1 row affected (0.00 sec)
mysql> select * from
tt;

+------------+

| num |

+------------+

| 234563.900 |

+------------+

1 row in set (0.00 sec)
mysql> insert into tt(num) values(2345623.2);

ERROR 1264 (22003): Out of range value for column 'num' at row 1

mysql> insert into tt(num) values(234563.2);

Query OK, 1 row affected (0.00 sec)
mysql> select
* from tt;

+------------+

| num |

+------------+

| 234563.900 |

| 234563.200 |

+------------+

2 rows in set (0.00 sec)
mysql> insert into tt(num) values(2.8);

Query OK, 1 row affected (0.00 sec)
mysql> select
* from tt;

+------------+

| num |

+------------+

| 234563.900 |

| 234563.200 |

| 2.800 |

+------------+

3 rows in set (0.00 sec)

FLOAT(M,D)或REAL(M,D)或DOUBLE PRECISION(M,D)。这里，“(M,D)”表示该值一共显示M位整数，其中D位位于小数点后面。

例如，定义为FLOAT(7,4)的一个列可以显示为-999.9999。MySQL保存值时进行四舍五入，因此如果在FLOAT(7,4)列内插入999.00009，近似结果是999.0001。