mysql提示used in key specification without a key length
来源：程序员人生 发布时间：2014-03-03 05:24:20 阅读次数：5623次
今天最火软件站小编在写一个PHP小东西的时候遇到了一个问题，设计数据库类型的时候提示：xxxx used in key specification without a key length，百思不得其解，后来在网上找到关于这个问题的答案。
原来Mysql数据库对于BLOB/TEXT这样类型的数据结构只能索引前N个字符。所以这样的数据类型不能作为主键，也不能是UNIQUE的。所以要换成VARCH,但是VARCHAR类型的大小也不能大于255，当VARCHAR类型的字段大小如果大于255的时候也会转换成小的TEXT来处理。所以也同样有问题。
官方的英文解释如下：
- The error happens because MySQL can index only the first N chars of a BLOB or TEXT column. So The error mainly happen when there is a field/column type of TEXT or BLOB or those belongs to TEXT or BLOB types such as TINYBLOB, MEDIUMBLOB, LONGBLOB, TINYTEXT, MEDIUMTEXT, and LONGTEXT that you try to make as primary key or index. With full BLOB or TEXT without the length value, MySQL is unable to guarantee the uniqueness of the column as it’s of variable and dynamic size. So, when using BLOB or TEXT types as index, the value of N must be supplied so that MySQL can determine the key length. However, MySQL doesn’t support a key length limit on TEXT or BLOB. TEXT(88) simply won’t work.
- The error will also pop up when you try to convert a table column from non-TEXT and non-BLOB type such as VARCHAR and ENUM into TEXT or BLOB type, with the column already been defined as unique constraints or index. The Alter Table SQL command will fail.
- The solution to the problem is to remove the TEXT or BLOB column from the index or unique constraint, or set another field as primary key. If you can’t do that, and wanting to place a limit on the TEXT or BLOB column, try to use VARCHAR type and place a limit of length on it. By default, VARCHAR is limited to a maximum of 255 characters and its limit must be specified implicitly within a bracket right after its declaration, i.e VARCHAR(200) will limit it to 200 characters long only.
- Sometimes, even though you don’t use TEXT or BLOB related type in your table, the Error 1170 may also appear. It happens in situation such as when you specify VARCHAR column as primary key, but wrongly set its length or characters size. VARCHAR can only accepts up to 256 characters, so anything such as VARCHAR(512) will force MySQL to auto-convert the VARCHAR(512) to a SMALLTEXT datatype, which subsequently fail with error 1170 on key length if the column is used as primary key or unique or non-
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