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MySQL中char与varchar的区别

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Jun 07, 2016 pm 04:24 PM

charmysqlvarvarcharthe difference

char和varchar类型相似，但是它们的存储和检索方式不同。在MySQL5.0.3，它们在最大长度和是否保留尾部空格也不相同。 char和varchar定义了字符串的最大长度。例如，char(30)可以放30个字符。 char列的长度在你建表的时候，就是你定义的固定长度。值的范围是0

char和varchar类型相似，但是它们的存储和检索方式不同。在MySQL5.0.3，它们在最大长度和是否保留尾部空格也不相同。

char和varchar定义了字符串的最大长度。例如，char(30)可以放30个字符。

char列的长度在你建表的时候，就是你定义的固定长度。值的范围是0~255。char值存储的时候，右部以空格来填充到指定的长度。当检索char的时候，会自动去掉尾部的空格。

varchar列是变长的。在MySQL5.0.3之前可以定义的长度是0~255,之后可以定义到0~65535。在MySQL5.0.3及以后的版本中varchar列的有效长度，受行的最大值和使用的字符限制。

对比char，varchar，加上前缀的数据长度是当做一个字节还是两个字节存储。这个前缀的长度表明了值的字节数。如果值不超过255的话，这个列用一个字节的长度，如果这个值可能超过255个字节，那么就使用两个字节的长度。

如果严格的SQL模式没有启用，你分配了一个char或者varchar列超出了列的最大长度，那么这个值就会被截短以填充列，这时候就会产生一个警告。对于截短的非空格字符，你可以设置严格的SQL模式，来产生一个错误。

对于varchar列来说，SQL模式在启用的时候，大量的尾部空格在插入之前都会被截短，产生一个警告。对于char列来说，不管SQL模式是否被启用，都会静默地去掉插入值的过量的尾部空格。

varchar值存储的时候是不会填充的。如何处理尾部空格，取决于版本。在MySQL5.0.3中，当值存储和检索的时候，保留尾部空格，和标准的SQL保持一致。在MySQL5.0.3之前，值被保存到varchar列的时候，尾部空格会被剔除。这样也就意味着检索值的时候，也不存在这些空格。

在MySQL5.0.3之前，如果你需要一个不去掉尾部空格的数据类型的时候，要考虑BLOB或者TEXT类型。同样当你存储加密数据或者压缩数据的时候，也要使用BLOB列，而不是char或者varchar列，来避免需要改变值的时候，尾部空格的潜在问题。

下面的表格来说明char和varchar的存储方式和显示结果的差别。（假设列用的很简单的字符，使用latin1编码）

MySQL中char与varchar的区别

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