UTF-8: General? Bin? Unicode?

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.

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You should also be aware of the fact, that with utf8_general_ci when using a varchar field as unique or primary index inserting 2 values like 'a' and 'á' would give a duplicate key error.

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utf8_bin compares the bits blindly. No case folding, no accent stripping.

utf8_general_ci compares one codepoint with one codepoint. It does case folding and accent stripping, but no 2-character comparisons; for example: 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.

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Accepted answer is outdated.

If you use MySQL 5.5.3+, use utf8mb4_unicode_ci instead of utf8_unicode_ci to ensure the characters typed by your users won't give you errors.

utf8mb4 supports emojis for example, whereas utf8 might give you hundreds of encoding-related bugs like:

Incorrect string value: ‘\xF0\x9F\x98\x81…’ for column ‘data’ at row 1

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Really, I tested saving values like 'é' and 'e' in column with unique index and they cause duplicate error on both 'utf8_unicode_ci' and 'utf8_general_ci'. You can save them only in 'utf8_bin' collated column.

And mysql docs (in http://dev.mysql.com/doc/refman/5.7/en/charset-applications.html) suggest into its examples set 'utf8_general_ci' collation.

[mysqld]
character-set-server=utf8
collation-server=utf8_general_ci

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