With the features described in the last paragraph you can write truly database independent applications. But MDB tries to go one step further: It allows you to define your schemas in XML. A manager converts this schema into the necessary SQL statements for each RDBMS. This means that you can use the same schema for any of the supported RDBMS. The examples for this section can be found in the xml_schema directory.
利用在上个段落中描述的特性，你能编写真正的数据库独立的程序。但是 MDB 尝试向前更加迈出一步：它允许你用 XML 定义你的 schema。一个管理器把这种 schema 转换为给每种 RDBMS 的必要的 SQL 语句。这意味着你能对所有支持的 RDBMS 使用相同的 schema。本节的例子能够在 xml_schema 目录中找到。
We will now write an XML schema file from scratch. First we must define an XML document. The database definition is contained within a database tag. The name of the database is defined using the name tag. The create tag tells the manager if the database should be created if it does not yet exist. If you split up your schema into several files you will only need to set create to 1 in the file you will submit first to the manager.
我们现在将从头编写一个 XML schema 文件。首先，我们必须定义一个 XML 文档。数据库定义是包含在一个 database 标签之中的。数据库的名字是使用 name 标签定义的。create 标签告诉管理器数据库是否需要在它不存在的时候被创建。如果你把你的 schema 文件分割成好几个文件你你首先提交给管理器的那个文件中把 create 设置为 1。
<?xml version="1.0" encoding="ISO-8859-1" ?>
<database>
<name>auth</name>
<create>1</create>
</database>
As you may have guessed from the database name auth the purpose of this database is to store user data for a simple authentication application. Listing 2 defines a table in which we can store the user data.
可能你已经从数据库名 auth 猜出了这个数据库的目的是用于储存简单的验证程序的用户数据。Listing 2 定义了在其中我们能储存用户数据的表。
Listing 2
<table>
<name>users</name>
<declaration>
<field>
<name>user_id</name>
<type>integer</type>
<notnull>1</notnull>
<unsigned>1</unsigned>
<default>0</default>
</field>
<field>
<name>handle</name>
<type>text</type>
<length>20</length>
<notnull>1</notnull>
<default></default>
</field>
<field>
<name>is_active</name>
<type>boolean</type>
<notnull>1</notnull>
<default>N</default>
</field>
</declaration>
</table>
As you can see, things can get a bit lengthy here as to be expected when using XML. No need to worry: We are working on a browser based tool called MDB_frontend that will make this process much easier. I will talk about this project further down into this article a bit more. Hopefully, the advantage of this pretty verbose representation of the table is that things are somewhat self explanatory. The table in the last example is called users and we have defined 3 fields: user_id of type integer, handle of type text and is_active of type boolean. Remember that MDB handles the type abstraction for you if you pass it the necessary metadata as shown in the previous section. You also need not to worry about what MDB maps these types to in your RDBMS. The other tags you can use in each of the field declarations are optional: length, notnull, unsigned and default.
如你能看到的，如使用 XML 时可以预期的，东西变得有一些冗长。不用担心：我们有一个基于浏览器的工具称为 MDB_frontend 使得这个过程更加简单。我将在这篇文章的后面谈论这个工程。可能这极其详细地表格描述的优点是非常明显。前面例子中的表格被称为 users 并且我们定义了 3 个域：类型为整数的 user_id，类型为文本的 handle 和类型为逻辑型的 is_active。记住如果你如前一节那样传递了必要的元数据 MDB 为你处理类型抽象。你还不需要 MDB 把这些类型映射为你的 RDBMS 中的什么。在每个域声明中还能使用的其他标签是可选的：length，notnull，unsigned 和 default。
The next thing that we now need to do is to ensure that the user_id is unique by placing the proper index on the user_id field. The index definition goes within the declaration tag (Listing 3).
下一件我们现在需要做的事情是通过在 user_id 域放置恰当的索引确保 user_id 是唯一的。索引定义就在声明标签之内（Listing 3）。
Listing 3:
<table>
<name>users</name>
<declaration>
<index>
<unique>1</unique>
<name>user_id_index</name>
<field>
<name>user_id</name>
<sorting>ascending</sorting>
</field>
</index>
</declaration>
</table>