一次编写，随处运行

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>

The definition in listing 3 would create a unique ascending index named user_id_index on the field user_id. Of course, we could have specified more than one field in the index definition by simply adding another field tag. What we are still missing now is a sequence to generate unique user id's for us:
在 listing 3 中的定义在域 user_id 中创建一个唯一的上升排序的名为 user_id_index 的索引。当然，我们可以简单地添加另外一个域标签在索引定义中指定多于一个的域。我们现在仍然没有提到的是为我们产生唯一的用户 id 的序列。

<sequence>
<name>users_user_id</name>
<start>1</start>
<on>
<table>users</table>
<field>user_id</field>
</on>
</sequence>

The last example is pretty mind blowing. Going through line by line we see that we first open a sequence tag followed by a name tag which specifies the name of the sequence. This is followed by a start tag that defines the initial value of the sequence. Now, we open an optional on tag. Here we need to set a specific field within a table. This information is used by the manager to set the value of the sequence to the maximum value in the user_id field of the users table. If the users table is empty the value specified in the start tag is used instead. Please note that the value specified in the start tag is the first value that will be returned if you call MDB::nextId().
上一个例子非常的绕弯。一行行看过来，我们看到首先打开一个 sequence 标签，跟着一个指定序列名字的 name 标签。这之后跟着一个定义序列初始值的 start 标签。现在，我们打开一个可选的 on标签。这儿我们需要设置一个表中的指定域。这个信息是管理器用来把序列的值设置为 users 表的 user_id 域的最大值。如果 users 表是空的，作为替代使用的是 start 标签中指定的值。请注意在 start 标签中指定的值是我们调用 MDB::nextId() 返回的第一个值。

Of course, you can also initialize a table with any values. For example you may want to initialize the above table with a maintenance user that you always want to include with your application. To do this we need to add an initialization tag to the table tag. Listing 4 defines one row after another enclosed with an insert tag.
当然，你也能使用任何值初始化表。例如你可能想要用你总是想要包含在你的程序中的管理用户来初始化前面的表格。为了这么做，我们需要把一个 initialization 标签添加给 table 标签。Listing 4 定义了一在另外一用 insert 标签包括的行之后的行。

Listing 4

<table>
<name>users</name>
<initialization>
<insert>
<field>
<name>user_id</name>
<value>1</value>
</field>
<field>
<name>handle</name>
<value>default</value>
</field>
<field>
<name>is_active</name>
<value>Y</value>
</field>
</insert>
</initialization>
</table>

As you can see from the last example all we have to do is to define a value for each field of the table. We now have the necessary basics to create an XML schema for MDB. The next step is to pass this schema file to the MDB manager.
如你从上个例子中能看到的那样，所有我们需要做的就是给表的每个域设定值。我们现在已经知道了必要的基础知识来创建一个 MDB 的 XML schema。下一步是把这个 schema 文件传递给 MDB 管理器。

$manager = new MDB_Manager;
$input_file = 'auth.schema';
// we do not have to connect to a specify a specific database at this time
$dsn = "mysql://$user:$pass@$host";
$manager->connect($dsn);
$manager->updateDatabase($input_file, $input_file. '.before');

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