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.

Now, 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’. Now, 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 ‘ö’) and multiple-character mappings (such as the rule that ‘ö’ = ‘OE’ in one of the two German collations).