Using attribute as a set of data and parsing this data

Question

Hi everyone,
I present below a (little) challenge with the use of attributes in XQuery. Consider the following XML document:
<showroom>
<car id="1">
<name>Clio</name>
<family>Renault</family>
</car>
<car id="n">
<name>Q5</name>
<family>Audi</family>
</car>
</showroom>
Each client of the showroom is identified by a unique Id (e.g., 1,2,3,....). Then, i want to add some data to my XML document which should represent, for each car, the set of all clients' Id that have rented this car.
The desired XML document could look like this:
<showroom>
<car id="1">
<name>Clio</name>
<family>Renault</family>
+*<history>*+
+*<client id="2"/>*+
+*<client id="23"/>*+
+*</history>*+
</car>
<car id="n">
<name>Q5</name>
<family>Audi</family>
+*<history>*+
+*<client id="56"/>*+
+*</history>*+
</car>
</showroom>
However, by using elements in order to represent the rental historic, the document's size will increase significantly.
As i use only the clients' id, are there any way to represent the history data using only attributes ?+.
In the following document, the clients' id are represented, as a set, through the attribute +@history+:
<showroom>
<car id="1" history="2,23">
<name>Clio</name>
<family>Renault</family>
</car>
<car id="n" history="56">
<name>Q5</name>
<family>Audi</family>
</car>
</showroom>
However, are there any way to check if a given car of id='x' has been rented by a client of Id='y' ?+.
Does someone know how to represent the history data (sequence of clients' id), using only attributes, and how to look for a given client's id as explained above ?
Please suggest. I appreciate your help.

Answer 1

The two following documents represent examples of the showroom data and the clients' data respectively:
First document:
<showroom>
<car id="1">
<name>Clio</name>
<family>Renault</family>
</car>
<car id="n">
<name>Q5</name>
<family>Audi</family>
</car>
</showroom>
Second document:
<clients>
<client id="2">
<name>...</name>
<firstname>...</firstname>
<driverLicence serialNumber="2156311"/>
</client>
</clients>
The DTDs of these documents are defined as follows:
The showroom DTD:
<!ELEMENT showroom (car)*>
<!ELEMENT car (name,family)>
<!ELEMENT name (#PCDATA)>
<!ELEMENT family (#PCDATA)>
<!ELEMENT N3 (#PCDATA)>
<!ATTLIST car id ID #REQUIRED>
The clients DTD:
<!ELEMENT clients (client)*>
<!ELEMENT client (name,firstname,driverLicence)>
<!ELEMENT name (#PCDATA)>
<!ELEMENT firstname (#PCDATA)>
<!ELEMENT driverLicence EMPTY>
<!ATTLIST client id ID #REQUIRED>
<!ATTLIST driverLicence serialNumber ID #REQUIRED>
Whenever a car is rented, I must add some information either in the same showroom document (it will be better for me) or in another document which relies rented car id with the id of the client which rented this car.
Thanks in advance for your help.