MySQL学习笔记12:数据类型
2015-10-19 21:22
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MySQL学习笔记12:数据类型
字符型数据
字符型数据可以使用定长或变长的字符串来实现,其不同点在于固定长度的字符串使用空格向右填充,以保证占用同样的字节数;变长字符串不需要向右填充,并且所有字节数可变。char 定长型,字节长度固定,最大长度为255字节,可以使用char(20)这种方式限定长度,查询速度快,但不灵活
varchar 变长型,字节长度随数据量变化,最多可存储65535字节,varchar(20)可以限定长度,查询速度不及char,但使用灵活
如果需要存储文本或者文档,需要使用text类型数据,包括四种
tinytext 255字节
text 65535字节
mediumtext 16777215字节
longtext 4294967295字节
以上三种在进行分组排序或者比较的时候只采用前1024字节进行比较
字符集
对于拉丁语系,比如英语,其中每个字母只需要1个字节来存储;而其他一些语言,比如中文,包含了大量字符,每个字符的存储需要多个字节,因此这类字符集被称为多字符集
在mysql中,可以使用SHOW CAHRACTER SET语句查看字符集
mysql> SHOW CHARACTER SET; +----------+-----------------------------+---------------------+--------+ | Charset | Description | Default collation | Maxlen | +----------+-----------------------------+---------------------+--------+ | big5 | Big5 Traditional Chinese | big5_chinese_ci | 2 | | dec8 | DEC West European | dec8_swedish_ci | 1 | | cp850 | DOS West European | cp850_general_ci | 1 | | hp8 | HP West European | hp8_english_ci | 1 | | koi8r | KOI8-R Relcom Russian | koi8r_general_ci | 1 | | latin1 | cp1252 West European | latin1_swedish_ci | 1 | | latin2 | ISO 8859-2 Central European | latin2_general_ci | 1 | | swe7 | 7bit Swedish | swe7_swedish_ci | 1 | | ascii | US ASCII | ascii_general_ci | 1 | | ujis | EUC-JP Japanese | ujis_japanese_ci | 3 | | sjis | Shift-JIS Japanese | sjis_japanese_ci | 2 | | hebrew | ISO 8859-8 Hebrew | hebrew_general_ci | 1 | | tis620 | TIS620 Thai | tis620_thai_ci | 1 | | euckr | EUC-KR Korean | euckr_korean_ci | 2 | | koi8u | KOI8-U Ukrainian | koi8u_general_ci | 1 | | gb2312 | GB2312 Simplified Chinese | gb2312_chinese_ci | 2 | | greek | ISO 8859-7 Greek | greek_general_ci | 1 | | cp1250 | Windows Central European | cp1250_general_ci | 1 | | gbk | GBK Simplified Chinese | gbk_chinese_ci | 2 | | latin5 | ISO 8859-9 Turkish | latin5_turkish_ci | 1 | | armscii8 | ARMSCII-8 Armenian | armscii8_general_ci | 1 | | utf8 | UTF-8 Unicode | utf8_general_ci | 3 | | ucs2 | UCS-2 Unicode | ucs2_general_ci | 2 | | cp866 | DOS Russian | cp866_general_ci | 1 | | keybcs2 | DOS Kamenicky Czech-Slovak | keybcs2_general_ci | 1 | | macce | Mac Central European | macce_general_ci | 1 | | macroman | Mac West European | macroman_general_ci | 1 | | cp852 | DOS Central European | cp852_general_ci | 1 | | latin7 | ISO 8859-13 Baltic | latin7_general_ci | 1 | | utf8mb4 | UTF-8 Unicode | utf8mb4_general_ci | 4 | | cp1251 | Windows Cyrillic | cp1251_general_ci | 1 | | utf16 | UTF-16 Unicode | utf16_general_ci | 4 | | cp1256 | Windows Arabic | cp1256_general_ci | 1 | | cp1257 | Windows Baltic | cp1257_general_ci | 1 | | utf32 | UTF-32 Unicode | utf32_general_ci | 4 | | binary | Binary pseudo charset | binary | 1 | | geostd8 | GEOSTD8 Georgian | geostd8_general_ci | 1 | | cp932 | SJIS for Windows Japanese | cp932_japanese_ci | 2 | | eucjpms | UJIS for Windows Japanese | eucjpms_japanese_ci | 3 | +----------+-----------------------------+---------------------+--------+ 39 rows in set (0.00 sec)
其中Maxlen列大于1则代表此行为多字符集,比如gb2312,存储一个汉字最多需要2个字节,utf8最多则需要3个
数值型数据
整数型数据
![](http://images.cnitblog.com/blog/476392/201301/22003109-b864afbdfff0483db88fa6a548752577.png)
浮点型数据
![](http://images.cnitblog.com/blog/476392/201301/22003354-b13cb211d27a4b6b8a2613c9a0bfe321.png)
数值型数据可以定义为unsigned,对整数而已可以改变其范围,对浮点数而言可以限定其不能为负数
时间数据
![](http://images.cnitblog.com/blog/476392/201301/22003822-890ad09af312404c83be3a7ee88f4b91.png)
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