8: General? Bin? Unicode?

 

I'm trying to figure out what collation I should be using for various types of data. 100% of the content I will be storing is user-submitted.

My understanding is that I should be using UTF-8 General CI (Case-Insensitive) instead of UTF-8 Binary. However, I can't find a clear a distinction between UTF-8 General CI and UTF-8 Unicode CI.

  • Should I be storing user-submitted content in UTF-8 General or UTF-8 Unicode CI columns?
  • What type of data would UTF-8 Binary be applicable to?

    • In general, utf8_general_ci is faster than utf8_unicode_ci, but less correct.

    Here is the difference:

    For any Unicode character set, operations performed using the _general_ci collation are faster than those for the _unicode_ci collation . For example, comparisons for the utf8_general_ci collation are faster, but slightly less correct, than comparisons for utf8_unicode_ci. The reason for this is that utf8_unicode_ci supports mappings such as expansions; that is, when one character compares as equal to combinations of other characters. For example, in German and some other languages “ß” is equal to “ss”. utf8_unicode_ci also supports contractions and ignorable characters. utf8_general_ci is a legacy collation that does not support expansions, contractions, or ignorable characters. It can make only one-to-one comparisons between characters.

    Quoted from: http://dev.mysql.com/doc/refman/5.0/en/charset-unicode-sets.html

    For more detailed explanation, please read the following post from MySQL forums: http://forums.mysql.com/read.php?103,187048,188748

    As for utf8_bin: Both utf8_general_ci and utf8_unicode_ci perform case-insensitive comparison. In constrast, utf8_bin is case-sensitive (among other differences), because it compares the binary values of the characters.

    你还应该知道这样一个事实：使用utf8_general_ci时，如果使用varchar字段作为唯一索引或主索引，插入2个值（如'a'和'á'）会导致重复键错误。

  • utf8_bin compares the bits blindly. No case folding, no accent stripping.
  • utf8_general_ci compares one byte with one byte. It does case folding and accent stripping, but no 2-character comparisions: ij is not equal ij in this collation.
  • utf8_*_ci is a set of language-specific rules, but otherwise like unicode_ci . Some special cases: Ç , Č , ch , ll
  • utf8_unicode_ci follows an old Unicode standard for comparisons. ij = ij , but ae != æ
  • utf8_unicode_520_ci follows an newer Unicode standard. ae = æ

    • See collation chart for details on what is equal to what in various utf8 collations.

    utf8 , as defined by MySQL is limited to the 1- to 3-byte utf8 codes. This leaves out Emoji and some of Chinese. So you should really switch to utf8mb4 if you want to go much beyond Europe.

    The above points apply to utf8mb4 , after suitable spelling change. Going forward, utf8mb4 and utf8mb4_unicode_520_ci are preferred.

  • utf16 and utf32 are variants on utf8; there is virtually no use for them.
  • ucs2 is closer to "Unicode" than "utf8"; there is virtually no use for it.
    • 链接地址: http://www.djcxy.com/p/66358.html

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