Character-based collation in Derby

A character set is a set of symbols and encodings. Character data types are represented as Unicode 2.0 sequences in Derby.

How collation works in Derby

Derby supports a wide range of character sets and encodes all of the character sets by using the Unicode support provided by the java.lang.Character class in the Java Virtual Machine (JVM) in which the Derby database runs. See the Java API documentation for the java.lang.Character class for the exact level of Unicode Standard that is supported.

A collation is a set of rules for comparing characters in a character set. In Derby the collation rules affect comparisons of the CHAR and VARCHAR data types. Collation rules also affect how the LIKE Boolean operator processes the CHAR, VARCHAR, CLOB, and LONG VARCHAR data types.

The default Derby collation rule is based on the binary Unicode values of the characters. So a character is greater than (<), equal to (=), or less than (>) another character based on the numeric comparison of the Unicode values. This rule allows for very efficient comparisons of character strings.

Derby also supports the ability to define collation rules that are appropriate to a territory, and is referred to as territory-based collation. You can specifically set the territory of a database when you create the database. If you do not specify a territory, Derby uses the default territory of the JVM in which the database is created. Each JVM can support many territories that are independent from the default territory for the JVM. Collation support for these additional territories is provided through the java.text.RuleBasedCollator class and the set of rules for these territories. Refer to the JVM specification for details of how these rules are used to provide territory specific collation. Derby currently supports only running those rules that can be loaded dynamically from the running JVM based on the territory attribute. Overrides to these rules by the user are not supported.

Territory-based collation does add extra processing overhead to all character-based comparison operations.

When you create a Derby database, the attributes that you set determine the collation that is used with all of character data in the database. For example:
Table 1. The create database attributes that control collation
Example create commands Collation is driven by
jdbc:derby:abcDB;create=true Unicode codepoint collation (UCS_BASIC), which is the default collation for Derby databases.
jdbc:derby:abcDB;create=true;territory=es_MX Unicode codepoint collation (UCS_BASIC). The collation attribute is not set.
jdbc:derby:abcDB;create=true;collation=TERRITORY_BASED The territory of the JVM, since the territory attribute is not set.
Tip: To determine the territory of the JVM, run Locale.getDefault().
jdbc:derby:abcDB;create=true;territory=es_MX;collation=TERRITORY_BASED The territory attribute.

The territory-based collation in Derby affects how the CHAR and VARCHAR data types are compared. Specifying territory-based collation also impacts how the LIKE Boolean operator processes CHAR, VARCHAR, CLOB, and LONG VARCHAR data. Derby supports the territories that Java supports.

Collation examples

With Unicode codepoint collation (UCS_BASIC), the numerical values of the Unicode encoding of the characters is used directly for ordering. For example, the FRUIT table contains the NAME column that uses the VARCHAR(20) data type. The contents of the NAME column are:
  • orange
  • apple
  • Banana
  • Pineapple
  • Grape
UCS_BASIC collation sorts all lower case letters before upper case letters. The statement SELECT * FROM FRUIT ORDER BY NAME returns:
  • apple
  • orange
  • Banana
  • Grape
  • Pineapple
If the database is created with the territory attribute set to en_US (English language, United States country code), and the collation attribute set to TERRITORY_BASED , the results of the statement SELECT * FROM FRUIT ORDER BY NAME returns:
  • apple
  • Banana
  • Grape
  • orange
  • Pineapple
The collation set for the database also impacts comparison operators on character data types. For example, the statement SELECT * FROM FRUIT WHERE NAME > Banana returns:

      UCS_BASIC collation       Territory-based collation
                Grape                                Grape
                Pineapple                           orange
                                                         Pineapple

Parent topic: Localizing Derby