mysql char,varcher,text的区别与选用原则_MySQL
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mysql char,varcher,text的区别与选用原则 mysql中char,varchar,text有什么使用区别呢？先看看他们的共同点：都可以用来存储字符类型的数据。char,varchar都可以指定最大的字符长度，text不可以。 比如,username char(10) , weibo vachar(200)这是在定义表的时候使用。都可以定义最大的长度。 区别：以上面的语句为例，username是char类型的数据，最大字符长度为10个字符，如果输入不满10个字符时，后面会自动被填充空格，也就是说始终会占用10个字符的长度。如果内容被取出之后，后面的空格将被自动清除（即使存入是后面带有空格，取出之后也会不空格给去掉，例如“abcd ”，后面有两个空格，最后取出之后也空格就没有了'abcd'） 而varchar类的是变长的，比如上例，200个最大长度，其中会有一到两个字节用来记录战用的长度。比如存进去100个字符，最后只会占有100个长度多一点（多的一连个记录占用空间的），而不是在后面补充空格占用所有长度。如果存进去后面有空格，那么后面的空格也会占用长度，取出之后空格还有事结果的一部分，这些是char和varchar之间的区别。 text主要是存储字符串，和varchar的长度相同，text不可以设置默认值，对索引的支持还不是很好或者就不支持。char的速度要比varhcar快速 使用原则：协调空间和时间的关系，以微博为例， 如果存储的是用户名的话，使用char，速度快，占用空间不是很大，空间使用率较高，比如最多10个字符，如果是存储微薄，有的用户微薄会占用100多个字符，而有的微薄最多占用10个字符，并且并不是每个用户只发一条微薄，如果使用char的话浪费率太高，空间浪费太大，使用text速度也跟不上，是用varchar比较好，空间是变长的，并且速度也算可以。现在的硬件的价格相对便宜，尽量让空间换时间，提高系统的效率与吞吐量，不过也要做到一定的度。需要根据具体情况具体分析考虑。 作者 com360 bitsCN.com
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