MySQL基础知识02数据类型(二)数值相关类型

标签：sql基础   ted   基础知识   prim   style   max   基础   ima   key   

1.INT

MySQL支持5中整数类型,每个整数类型又可以分为有符号整数和无符号整数。

1.1.2. DECIMAL(NUMERIC)

MySQL使用DECIMAL表示定点数，NUMERIC是DECIMAL的一个别名。

DECIMAL(5,2)表示这个定点数类型的整数和小数部分总共5位，其中3位是整数，2位是小数。

DECIMAL的总位数最多65位，超出65则报错。

mysql> create table n3 ( id numeric(66) primary key);

ERROR 1426 (42000): Too-big precision 66 specified for ‘id‘. Maximum is 65.

创建一个包含62位整数加3位小数的定点数类型的列的数据表。

mysql> create table n2 ( id numeric(65,3) primary key);

Query OK, 0 rows affected (0.02 sec)

mysql> show create table n2;

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

| Table | Create Table                                                                                                   |

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

| n2    | CREATE TABLE `n2` (

  `id` decimal(65,3) NOT NULL,

  PRIMARY KEY (`id`)

) ENGINE=InnoDB DEFAULT CHARSET=latin1 |

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

1 row in set (0.00 sec)

在DECIMAL(65,3)类型中，小数部分超出3位时则自动截取为3位小数。

mysql> insert into n2 values (12345678901234567890123456789012345678901234567890123456789012.345);

Query OK, 1 row affected (0.05 sec)

mysql> insert into n2 values (12345678901234567890123456789012345678901234567890123456789012.12345);

Query OK, 1 row affected, 1 warning (0.01 sec)

mysql> insert into n2 values (12345678901234567890123456789012345678901234567890123456789012);

Query OK, 1 row affected (0.01 sec)

mysql> select * from n2;

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

| id                                                                 |

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

| 12345678901234567890123456789012345678901234567890123456789012.000 |

| 12345678901234567890123456789012345678901234567890123456789012.123 |

| 12345678901234567890123456789012345678901234567890123456789012.345 |

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

3 rows in set (0.00 sec)

在DECIMAL(65,3)类型中，整数部分不允许超过62位。

mysql> insert into n2 values (12345678901234567890123456789012345678901234567890123456789012345);

ERROR 1264 (22003): Out of range value for column ‘id‘ at row 1

在使用DECIMAL类型时，如果不指定精度，则默认为DECIMAL(10,0)。

mysql> create table n7  ( id decimal primary key);

Query OK, 0 rows affected (0.03 sec)

mysql> show create table n7;

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

| Table | Create Table                                                                                                   |

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

| n7    | CREATE TABLE `n7` (

  `id` decimal(10,0) NOT NULL,

  PRIMARY KEY (`id`)

) ENGINE=InnoDB DEFAULT CHARSET=latin1 |

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

1 row in set (0.00 sec)

定点数的运算：

mysql> select cast( 123456789012345678901234567890.1234567890 as decimal(40,10) ) + cast(1.1111111111 as decimal(40,10) );

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

| cast( 123456789012345678901234567890.1234567890 as decimal(40,10) ) + cast(1.1111111111 as decimal(40,10) ) |

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

|                                                                   123456789012345678901234567891.2345679001 |

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

1 row in set (0.01 sec)

2.FLOAT/DOUBLE

MySQL提供了浮点数FLOAT和DOUBLE类型，其中FLOAT占用4个字节，DOUBLE占用8个字节。

FLOAT(5,2)表示有3位整数加上2位小数的浮点数。

浮点数的查询不能简单的相等比较：

mysql> create table n8 ( id float(10,3) primary key);

Query OK, 0 rows affected (0.03 sec)

mysql> show create table n8;

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

| Table | Create Table                                                                                                 |

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

| n8    | CREATE TABLE `n8` (

  `id` float(10,3) NOT NULL,

  PRIMARY KEY (`id`)

) ENGINE=InnoDB DEFAULT CHARSET=latin1 |

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

1 row in set (0.00 sec)

mysql> insert into n8 values ( 1234567.123);

Query OK, 1 row affected (0.01 sec)

mysql> select * from n8;

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

| id          |

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

| 1234567.125 |

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

1 row in set (0.00 sec)

mysql> select * from n8 where id=1234567.123;

Empty set (0.00 sec)

mysql> select * from n8 where id=1234567.125;

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

| id          |

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

| 1234567.125 |

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

1 row in set (0.00 sec)

mysql> select * from n8 where id=1234567.126;

Empty set (0.00 sec)

上述查询中，插入的原始数据是1234567.123，而查询到的实际是1234567.125。

MySQL基础知识02数据类型(二)数值相关类型

标签：sql基础   ted   基础知识   prim   style   max   基础   ima   key   

原文地址：http://www.cnblogs.com/coe2coe/p/7531273.html