If the only thing you are storing in the database is the XML data, what is to stop you from storing the file directly on the filesystem and bypassing the overhead of querying the database for the single field?

If backups are your concern, there are methods to backup a directory as well.

Usage of MySQL (or other RDBMS) implies there is a relational aspect to the data you are storing. If there's no relationship, this is probably not the tool you are looking for.

You could try using MySQL's mysqldump utility.

There is an option to make mysqldump export in XML

[redwards@lw-lts-155 ~]$ mysqldump --help | grep xml
  -X, --xml           Dump a database as well formed XML.

In your case, you could dump the table mydb.mytable like this:

mysqldump --xml mydb mytable > mytable.xml

EXAMPLE

For the given table

mysql> show create table groupby\G
*************************** 1. row ***************************
       Table: groupby
Create Table: CREATE TABLE `groupby` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `num` int(11) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=MyISAM AUTO_INCREMENT=17 DEFAULT CHARSET=latin1
1 row in set (0.01 sec)

mysql> select * from groupby;
+----+------------+
| id | num        |
+----+------------+
|  1 |  360370418 |
|  2 |  429187192 |
|  3 | 1064824747 |
|  4 | 1328623077 |
|  5 |  740702064 |
|  6 | 1071610692 |
|  7 |  428008187 |
|  8 |  279178462 |
|  9 |  360370418 |
| 10 |  429187192 |
| 11 | 1064824747 |
| 12 | 1328623077 |
| 13 |  740702064 |
| 14 | 1071610692 |
| 15 |  428008187 |
| 16 |  279178462 |
+----+------------+
16 rows in set (0.00 sec)

mysql>

just mysqldump to a textfile.

Here is what the XML actually looks like

<?xml version="1.0"?>
<mysqldump xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
<database name="eggyal">
        <table_structure name="groupby">
                <field Field="id" Type="int(11)" Null="NO" Key="PRI" Extra="auto_increment" Comment="" />
                <field Field="num" Type="int(11)" Null="YES" Key="" Extra="" Comment="" />
                <key Table="groupby" Non_unique="0" Key_name="PRIMARY" Seq_in_index="1" Column_name="id" Collation="A" Cardinality="16" Null="" Index_type="BTREE" Comment="" Index_comment="" />
                <options Name="groupby" Engine="MyISAM" Version="10" Row_format="Fixed" Rows="16" Avg_row_length="9" Data_length="144" Max_data_length="2533274790395903" Index_length="2048" Data_free="0" Auto_increment="17" Create_time="2012-11-26 22:39:22" Update_time="2012-11-26 22:39:22" Collation="latin1_swedish_ci" Create_options="" Comment="" />
        </table_structure>
        <table_data name="groupby">
        <row>
                <field name="id">1</field>
                <field name="num">360370418</field>
        </row>
        <row>
                <field name="id">2</field>
                <field name="num">429187192</field>
        </row>
        <row>
                <field name="id">3</field>
                <field name="num">1064824747</field>
        </row>
        <row>
                <field name="id">4</field>
                <field name="num">1328623077</field>
        </row>
        <row>
                <field name="id">5</field>
                <field name="num">740702064</field>
        </row>
        <row>
                <field name="id">6</field>
                <field name="num">1071610692</field>
        </row>
        <row>
                <field name="id">7</field>
                <field name="num">428008187</field>
        </row>
        <row>
                <field name="id">8</field>
                <field name="num">279178462</field>
        </row>
        <row>
                <field name="id">9</field>
                <field name="num">360370418</field>
        </row>
        <row>
                <field name="id">10</field>
                <field name="num">429187192</field>
        </row>
        <row>
                <field name="id">11</field>
                <field name="num">1064824747</field>
        </row>
        <row>
                <field name="id">12</field>
                <field name="num">1328623077</field>
        </row>
        <row>
                <field name="id">13</field>
                <field name="num">740702064</field>
        </row>
        <row>
                <field name="id">14</field>
                <field name="num">1071610692</field>
        </row>
        <row>
                <field name="id">15</field>
                <field name="num">428008187</field>
        </row>
        <row>
                <field name="id">16</field>
                <field name="num">279178462</field>
        </row>
        </table_data>
</database>
</mysqldump>

As for the DTD itself, you should look around StackOverflow and ServerFault

You could also XML dump just the structure using --no-data option

mysqldump --xml --no-data mydb mytable > mytable.xml

which produces

<?xml version="1.0"?>
<mysqldump xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
<database name="eggyal">
        <table_structure name="groupby">
                <field Field="id" Type="int(11)" Null="NO" Key="PRI" Extra="auto_increment" Comment="" />
                <field Field="num" Type="int(11)" Null="YES" Key="" Extra="" Comment="" />
                <key Table="groupby" Non_unique="0" Key_name="PRIMARY" Seq_in_index="1" Column_name="id" Collation="A" Cardinality="16" Null="" Index_type="BTREE" Comment="" Index_comment="" />
                <options Name="groupby" Engine="MyISAM" Version="10" Row_format="Fixed" Rows="16" Avg_row_length="9" Data_length="144" Max_data_length="2533274790395903" Index_length="2048" Data_free="0" Auto_increment="17" Create_time="2012-11-26 22:39:22" Update_time="2012-11-26 22:39:22" Collation="latin1_swedish_ci" Create_options="" Comment="" />
        </table_structure>
</database>
</mysqldump>

Take that output and scult it into a DTD