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
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*
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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