Using DISTINCT Data Type使用DISTINCT数据类型

Note: MySQL and Java DB currently do not support the DISTINCT SQL data type. 注意：MySQL和Java DB目前不支持DISTINCTSQL数据类型。Consequently, no JDBC tutorial example is available to demonstrate the features described in this section.因此，没有JDBC教程示例可用于演示本节中描述的功能。

The DISTINCT data type behaves differently from the other advanced SQL data types. DISTINCT数据类型的行为不同于其他高级SQL数据类型。Being a user-defined type that is based on one of the already existing built-in types, it has no interface as its mapping in the Java programming language. 作为一种基于现有内置类型之一的用户定义类型，它在Java编程语言中没有接口作为其映射。Instead, the standard mapping for a DISTINCT data type is the Java type to which its underlying SQL data type maps.相反，DISTINCT数据类型的标准映射是其底层SQL数据类型映射到的Java类型。

To illustrate, create a DISTINCT data type and then see how to retrieve, set, or update it. 举例来说，创建一个DISTINCT数据类型，然后查看如何检索、设置或更新它。Suppose you always use a two-letter abbreviation for a state and want to create a DISTINCT data type to be used for these abbreviations. 假设您总是使用两个字母的缩写来表示一个州，并希望创建一个用于这些缩写的DISTINCT数据类型。You could define your new DISTINCT data type with the following SQL statement:您可以使用以下SQL语句定义新的DISTINCT数据类型：

CREATE TYPE STATE AS CHAR(2);

Some databases use an alternate syntax for creating a DISTINCT data type, which is shown in the following line of code:某些数据库使用另一种语法来创建DISTINCT的数据类型，如以下代码行所示：

CREATE DISTINCT TYPE STATE AS CHAR(2);

If one syntax does not work, you can try the other. 如果一种语法不起作用，可以尝试另一种语法。Alternatively, you can check the documentation for your driver to see the exact syntax it expects.或者，您可以查看驱动程序的文档，以查看它所需的确切语法。

These statements create a new data type, STATE, which can be used as a column value or as the value for an attribute of a SQL structured type. 这些语句创建一个新的数据类型STATE，它可以用作列值或SQL结构化类型的属性值。Because a value of type STATE is in reality a value that is two CHAR types, you use the same method to retrieve it that you would use to retrieve a CHAR value, that is, getString. 因为STATE类型的值实际上是两种CHAR类型的值，所以可以使用与检索CHAR值相同的方法来检索它，即getString。For example, assuming that the fourth column of ResultSet rs stores values of type STATE, the following line of code retrieves its value:例如，假设ResultSet rs的第四列存储STATE类型的值，下面的代码行检索其值：

String state = rs.getString(4);

Similarly, you would use the method setString to store a STATE value in the database and the method updateString to modify its value.类似地，您将使用方法setString在数据库中存储STATE值，并使用方法updateString修改其值。