(mysql.info.gz) Charset-general

 
 10.1 Character Sets and Collations in General
 =============================================
 
 A *character set* is a set of symbols and encodings. A *collation* is a
 set of rules for comparing characters in a character set. Let's make
 the distinction clear with an example of an imaginary character set.
 
 Suppose that we have an alphabet with four letters: `A', `B', `a', `b'.
 We give each letter a number: `A' = 0, `B' = 1, `a' = 2, `b' = 3. The
 letter `A' is a symbol, the number 0 is the *encoding* for `A', and the
 combination of all four letters and their encodings is a *character
 set*.
 
 Suppose that we want to compare two string values, `A' and `B'.  The
 simplest way to do this is to look at the encodings: 0 for `A' and 1
 for `B'. Because 0 is less than 1, we say `A' is less than `B'. What
 we've just done is apply a collation to our character set. The
 collation is a set of rules (only one rule in this case): "compare the
 encodings." We call this simplest of all possible collations a *binary*
 collation.
 
 But what if we want to say that the lowercase and uppercase letters are
 equivalent? Then we would have at least two rules: (1) treat the
 lowercase letters `a' and `b' as equivalent to `A' and `B'; (2) then
 compare the encodings. We call this a *case-insensitive* collation.
 It's a little more complex than a binary collation.
 
 In real life, most character sets have many characters: not just `A'
 and `B' but whole alphabets, sometimes multiple alphabets or eastern
 writing systems with thousands of characters, along with many special
 symbols and punctuation marks. Also in real life, most collations have
 many rules: not just case insensitivity but also accent insensitivity
 (an "accent" is a mark attached to a character as in German `O"') and
 multiple-character mappings (such as the rule that `O"' = `OE' in one
 of the two German collations).
 
 MySQL 4.1 can do these things for you:
 
    * Store strings using a variety of character sets
 
    * Compare strings using a variety of collations
 
    * Mix strings with different character sets or collations in the
      same server, the same database, or even the same table
 
    * Allow specification of character set and collation at any level
 
 
 In these respects, not only is MySQL 4.1 far more flexible than MySQL
 4.0, it also is far ahead of other DBMSs. However, to use the new
 features effectively, you will need to learn what character sets and
 collations are available, how to change their defaults, and what the
 various string operators do with them.
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