Pure aggregate queries return single row results. What am I missing?

Thanks

Hibernate version: Hibernate 2.1.6

Mapping documents:
<hibernate-mapping>

<class name="com.robico.data.billing.InvoiceItemBO" table="invoice_item">

<id name="id" type="int" unsaved-value="0" >
<column name="item_id" sql-type="int(6) unsigned" not-null="true"/>
<generator class="identity"/>
</id>
<property name="charge">
<column name="charge" sql-type="double" not-null="true"/>
</property>
<property name="resourceUsageId" column="cdr_id" type="int" insert="false" update="false" not-null="true"/>
<many-to-one name="chargeSource"
class="com.robico.billing.asterisk.data.AsteriskChargeBO"
column="charge_id" cascade="none"/>
<many-to-one name="account"
class="com.robico.data.billing.AccountBO"
column="account_id" cascade="all"/>
<many-to-one name="resourceUsage"
class="com.robico.data.billing.ResourceUsageBO"
column="cdr_id" cascade="none"/>
</class>
</hibernate-mapping>

<hibernate-mapping>
<class name="com.robico.data.billing.ResourceUsageBO" table="cdr">
<id name="id" type="string" unsaved-value="0" >
<column name="uniqueID" sql-type="varchar(32)" not-null="true"/>
<generator class="native"/>
</id>
<property name="usageDate">
<column name="callDate" sql-type="datetime" not-null="true"/>
</property>
<property name="usageUnits" type="double" column="billableDuration" not-null="true"/>
<property name="resourceName" type="string" column="accountCode" not-null="false"/>
<component name="displayDetails"
class="com.robico.billing.asterisk.data.CdrDetails">
<property name="callCost" type="double" column="callCost" not-null="false"/>
<property name="callDisposition" type="string" column="callDisposition" not-null="false"/>
<property name="calledNumber" type="string" column="calledNumber" not-null="true"/>
<property name="callerId" type="string" column="callerID" not-null="false"/>
<property name="endDate" type="calendar" column="endDate" not-null="true"/>
</component>
</class>
<query name="com.robico.data.billing.ResourceUsageBO.getLatest">
from com.robico.data.billing.ResourceUsageBO as usage
where usage.id > ?
</query>

</hibernate-mapping>



Code between sessionFactory.openSession() and session.close():

String maxId = ( String )
session.createQuery( "select max(resourceUsageId) from com.robico.data.billing.InvoiceItemBO" ).uniqueResult();


Full stack trace of any exception that occurs:

net.sf.hibernate.NonUniqueResultException: query did not return a unique result: 11
at net.sf.hibernate.impl.AbstractQueryImpl.uniqueElement(AbstractQueryImpl.java:559)
at net.sf.hibernate.impl.AbstractQueryImpl.uniqueResult(AbstractQueryImpl.java:550)
at com.robico.billing.asterisk.AsteriskChargeManager.createInvoiceItems(AsteriskChargeManager.java:63)
at com.robico.billing.asterisk.AsteriskChargeManager$1.run(AsteriskChargeManager.java:47)
at java.util.TimerThread.mainLoop(Timer.java:512)
at java.util.TimerThread.run(Timer.java:462)


Name and version of the database you are using:

MySql 4.0.18

The generated SQL (show_sql=true):

Debug level Hibernate log excerpt:

Any suggestions?

Check the generated SQL