详解mysql int类型的长度值问题
作者：追逐_something
我的朋友海滨问我mysql在建表的时候int类型后的长度代表什么? 是该列允许存储值的最大宽度吗? 为什么我设置成int(1), 也一样能存10,100,1000呢.
当时我虽然知道int(1),这个长度1并不代表允许存储的宽度,但却没有一个合理的解释. 或者说对这个长度也没有真正的研究过到底代表什么, 平时都用int(11), 也不知道为什么要11位. 所以我在网上查阅了一些资料, 也仔细的看了mysql手册关于int data type的说法.
以下是每个整数类型的存储和范围(来自mysql手册)
| 类型 | 字节 | 最小值 | 最大值 |
| (带符号的/无符号的) | (带符号的/无符号的) |
||
| TINYINT | 1 | -128 | 127 |
| 0 | 255 | ||
| SMALLINT | 2 | -32768 | 32767 |
| 0 | 65535 | ||
| MEDIUMINT | 3 | -8388608 | 8388607 |
| 0 | 16777215 | ||
| INT | 4 | -2147483648 | 2147483647 |
| 0 | 4294967295 | ||
| BIGINT | 8 | -9223372036854775808 | 9223372036854775807 |
| 0 | 18446744073709551615 |
表格一共有四列分别表式:字段类型, 占用字节数, 允许存储的最小值, 允许存储的最大值.
我们拿int类型为例:
int类型, 占用字节数为4byte, 学过计算机原理的同学应该知道, 字节(byte)并非是计算机存储的最小单位, 还有比字节(byte)更小的单位, 也就是位(bit),一个位就代表一个0或1; 8个位组成一个字节; 一般字节用大写B来表示byte, 位用小写b来表示bit.
计算机存储单位的换算:
1B=8b
1KB=1024B
1MB=1024KB
那么根据int类型允许存储的字节数是4个字节, 我们就能换算出int UNSIGNED(无符号)类型的能存储的最小值为0, 最大值为4294967295(即4B=32b, 最大值即为32个1组成);
接下来我们再说说我们建表时的字段长度到底是怎么一回事.
CREATE TABLE `test` (
`id` INT( 11 ) NOT NULL AUTO_INCREMENT PRIMARY KEY ,
`number` INT( 5 ) NOT NULL
) ENGINE = MYISAM ;
以test表的number字段为例, 大家看到我建的是int(5)
mysql手册中这个长度/值用"M"来表示的. 细心的朋友应该有注意到过mysql手册上有这么一句话: M指示最大显示宽度。最大有效显示宽度是255。显示宽度与存储大小或类型包含的值的范围无关;
这句话看上去不太容易理解, 因为这里有个关键词容易让我们混淆, "最大显示宽度"我们第一反应是该字段的值最大能允许存放的值的宽度. 以为我们建了int(1),就不能存放数据10了, 其实不是这个意思.
这个M=5我们可以简单的理解成为, 我们建立这个长度是为了告诉MYSQL 数据库 我们这个字段的存储的数据的宽度为5位数, 当然如果你不是5位数(只要在该类型的存储范围之内)MYSQL也能正常存储, 这也就能解释以上标红的话.
我们把这个字段的"属性"修改为UNSIGNED ZEROFILL看一下效果.
我们看到现在我的number字段, 长度(M)=5, 属性=UNSIGNED ZEROFILL(无符号,用0来填充位数), 设置这个属性后我往表时插入数据,系统会自动把number字段M不够5位的在左侧用0来填充; 效果如下
手册上还有这么一句话"当 mysql 为某些复杂的联结(join)生成临时表时，你可能会遇到问题，因为在这种情况下，mysql 信任地认为所有的值均适合原始的列宽度". 这也让我不禁感叹这个宽度到底如何设置比较合适?
但有一点看完该文档你应该清楚的知道, 长度M与你存放的数值型的数的大小无关.
原文地址：【】
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