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 for
whether to distinguish lettercase, but also for whether to
distinguish accents (an “accent” is a mark attached
to a character as in German ‘Ö’),
and for multiple-character mappings (such as the rule that
‘Ö’ =
‘OE’ in one of the two German
collations).
MySQL 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, MySQL is far ahead of most other database
management systems. However, to use these features effectively,
you need to know what character sets and collations are available,
how to change the defaults, and how they affect the behavior of
string operators and functions.
