<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2" assembly="PersistentData" namespace="PersistentData">
  <class name="IPersistentObject" table="TBL_PERSISTENT_OBJECTS" abstract="true">
    <id name="id" type="string" length="18">
      <column name="ID"/>
      <generator class="PersistentData.PrefixIDGenerator, PersistentData"/>
    </id>
    <discriminator column="discr" type="string"/>
  </class>
</hibernate-mapping>

<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2" assembly="MedicalData" namespace="MedicalData.Logging">
  <subclass name="ChangeLogMessage" extends="PersistentData.IPersistentObject" discriminator-value="ChangeLogMessage">
    <join table="TBL_CHANGE_LOG">
      <key foreign-key="FK_CHANGE_LOG_MESSAGE" column="ID"/>
      <property name="objectID" column="objectID" not-null="true"/>
      <property name="type" column="type" type="ChangeType" not-null="true"/>
    </join>
  </subclass>
</hibernate-mapping>

select max(obj.id) 
from PersistentData.IPersistentObject as obj

select max(obj.id)
from MedicalData.ChangeLogMessage as obj

select max(changelogm0_.ID) as col_0_0_ 
from TBL_PERSISTENT_OBJECTS changelogm0_ 
   inner join TBL_CHANGE_LOG changelogm0_1_ on changelogm0_.ID=changelogm0_1_.ID
where changelogm0_.discr='ChangeLogMessage'