Unicode Collation Algorithm (UCA)

The Unicode Collation Algorithm (UCA) is an algorithm for sorting the entire Unicode character set.

It provides linguistically correct comparison, ordering, and case conversion. The UCA was developed as part of the Unicode standard. The UCA is implemented using the International Components for Unicode (ICU) open source library, developed and maintained by IBM.

Note The default UCA ordering sorts most characters in most languages into an appropriate order. However, because of the sorting and comparison variations between languages sharing characters, the UCA cannot provide proper sorting for all languages. For this purpose, ICU provides collation tailoring options to the UCA.

The UCA provides advanced comparison, ordering, and case conversion at a small cost in space and time.

The mapped form of a string is longer than the original string. The algorithm provides sophisticated handling of more complex characters.

Unlike the SQL Anywhere Collation Algorithm (SACA) the Unicode Collation Algorithm (UCA) is only for use with single-byte and UTF-8 character sets, and it separates each character into one or more attributes. For letters, these attributes are base character, accent, and case.

Non-letters typically have only one attribute, the base character.

UCA compares character strings as follows:

Compare the base characters. If one string of base characters differs from the other, then the comparison is complete. Accent and case are not considered.
If the database is accent sensitive, compare the accents. If the accents differ, then the comparison is complete. Case is not considered.
If the database is case sensitive, compare the case of each character.

The original string values are equal if and only if the base characters, accents, and case are the same for both strings.

Example

Suppose UCA is used to compare the strings in the first column of the table below. The subsequent columns describe the three attributes for each string. The base characters are identical; the words differ only in accents and case.

String	Base characters	Accents	Case
noel	noel	none, none, none, none	lower, lower, lower, lower
noël	noel	none, none, accent, none	lower, lower, lower, lower
Noel	noel	none, none, none, none	upper, lower, lower, lower
Noël	noel	none, none, accent, none	upper, lower, lower, lower

The following table shows the ordering that would occur in the four possible combinations of accent- and case-sensitivity using UCA:

Accent sensitive	Case sensitive	ORDER BY result	Explanation
N	N	Noel, noël, Noël, noel in any order	Accents ignored Case ignored All values considered equal Random order within set of four
Y	N	Noel, noel in any order, followed by noël, Noël in any order	No-accents before accents, so e before ë Case ignored, N and n are in random order within each set of two
N	Y	Noel, Noël in any order, followed by noël, noel in any order	Uppercase before lowercase, so N before n Accents ignored, e and ë are in random order within each set of two
Y	Y	Noel noel Noël noël	No-accents before accents, so e before ë Uppercase before lowercase, so N before n