diff --git a/Doc/course.xml b/Doc/course.xml
index c3f83e8324213513e24a72cfcf580fa17771e652..127b6558889dbe2a897a0ac17e77406fadd0456e 100644
--- a/Doc/course.xml
+++ b/Doc/course.xml
@@ -17345,20 +17345,167 @@ public class CreditCard extends BillingDetails {
linkend="tpcNotNullCardType"/> and <coref
linkend="tpcNotNullexpiration"/>.</para>
- <para>Abbildung: Tiefe Vererbungshierarchie</para>
-
<section xml:id="joinedSubclassRetrieve">
<title>Retrieving Objects</title>
- <para>On the database server side object retrieval becomes a more
- expensive operation:</para>
+ <para>On the database server side object retrieval results in a
+ more expensive operation. A query for the root class
+ <classname>inherit.joined.v1.BillingDetails</classname> <coref
+ linkend="joinedQueryBillingDetails"/> of our inheritance hierarchy
+ results in joining all three tables <code>BillingDetails</code>
+ <coref linkend="joinFromBillingDetails"/>,
+ <code>BankAccount</code> <coref linkend="joinFromBankAccount"/>
+ and <code>CreditCard</code> <coref
+ linkend="joinFromCreditCard"/>:</para>
+
+ <informaltable border="1">
+ <colgroup width="6%"/>
+
+ <colgroup width="94%"/>
+
+ <tr>
+ <td valign="top"><emphasis role="bold">Java</emphasis></td>
+
+ <td valign="top"><programlisting>package inherit.joined.v1;
+...
+public class RetrieveAll {
+...
+ final Query searchBilling = session.createQuery("<emphasis role="bold">from inherit.tpc.v1.BillingDetails</emphasis>" <co
+ xml:id="joinedQueryBillingDetails"/>);
+...</programlisting></td>
+ </tr>
+
+ <tr>
+ <td valign="top"><emphasis role="bold">Sql</emphasis></td>
+
+ <td><programlisting continuation="continues">Hibernate:
+ select
+ billingdet0_.id as id0_,
+ billingdet0_.created as created0_,
+ billingdet0_.number as number0_,
+ billingdet0_1_.bankName as bankName1_,
+ billingdet0_1_.swiftcode as swiftcode1_,
+ billingdet0_2_.cardType as cardType2_,
+ billingdet0_2_.expiration as expiration2_,
+ case
+ when billingdet0_1_.id is not null then 1
+ when billingdet0_2_.id is not null then 2
+ when billingdet0_.id is not null then 0
+ end as clazz_
+ from
+ <emphasis role="bold">BillingDetails</emphasis> billingdet0_ <co
+ xml:id="joinFromBillingDetails"/>
+ left outer join
+ <emphasis role="bold">BankAccount</emphasis> billingdet0_1_ <co
+ xml:id="joinFromBankAccount"/>
+ on billingdet0_.id=billingdet0_1_.id
+ left outer join
+ <emphasis role="bold">CreditCard</emphasis> billingdet0_2_ <co
+ xml:id="joinFromCreditCard"/>
+ on billingdet0_.id=billingdet0_2_.id </programlisting></td>
+ </tr>
+ </informaltable>
<qandaset role="exercise">
<qandadiv>
<qandaentry>
<question>
- <para/>
+ <para>Explain all integrity constraints of the Hibernate
+ generated schema. Is it able to implement the correct
+ constraints on database level corresponding to the
+ inheritance related <trademark
+ xlink:href="http://www.oracle.com/us/technologies/java">Java</trademark>
+ objects? On contrary: Are there possible database states
+ which do not correspond to the domain model's object
+ constraints? </para>
</question>
+
+ <answer>
+ <para>We take a look to the database schema:</para>
+
+ <programlisting>CREATE TABLE BillingDetails (
+ id bigint(20) NOT NULL AUTO_INCREMENT PRIMARY KEY <co
+ linkends="inheritJoinSqlJava-1"
+ xml:id="inheritJoinSqlJava-1-co"/>,
+ created datetime NOT NULL,
+ number varchar(32) NOT NULL
+);
+CREATE TABLE CreditCard (
+ id bigint(20) NOT NULL PRIMARY KEY <co linkends="inheritJoinSqlJava-2"
+ xml:id="inheritJoinSqlJava-2-co"/> REFERENCES <co
+ linkends="inheritJoinSqlJava-3"
+ xml:id="inheritJoinSqlJava-3-co"/> BillingDetails,
+ cardType int(11) NOT NULL,
+ expiration datetime NOT NULL
+);
+CREATE TABLE BankAccount (
+ id bigint(20) NOT NULL PRIMARY KEY <co linkends="inheritJoinSqlJava-4"
+ xml:id="inheritJoinSqlJava-4-co"/> REFERENCES <co
+ linkends="inheritJoinSqlJava-4"
+ xml:id="inheritJoinSqlJava-5-co"/> BillingDetails,
+ bankName varchar(255) NOT NULL,
+ swiftcode varchar(255) NOT NULL
+);</programlisting>
+
+ <calloutlist>
+ <callout arearefs="inheritJoinSqlJava-1-co"
+ xml:id="inheritJoinSqlJava-1">
+ <para>The table implementing the root class
+ <classname>inherit.joined.v1.BillingDetails</classname>
+ of the inheritance hierarchy will be referenced both
+ by <code>CreditCard</code> and
+ <code>BankAccount</code> datasets and thus requires a
+ key to become addressable. Moreover the corresponding
+ <classname>inherit.joined.v1.BillingDetails</classname>
+ class requires this attribute to be the primary key
+ anyway.</para>
+ </callout>
+
+ <callout arearefs="inheritJoinSqlJava-2-co"
+ xml:id="inheritJoinSqlJava-2">
+ <para>Each <code>CreditCard</code> portion of
+ attributes belongs to exactly one
+ <code>BillingDetails</code> instance and hence the id
+ must be unique.</para>
+ </callout>
+
+ <callout arearefs="inheritJoinSqlJava-3-co"
+ xml:id="inheritJoinSqlJava-3">
+ <para>As stated in <coref
+ linkend="inheritJoinSqlJava-2-co"/> each
+ <code>CreditCard</code> dataset must refer to its
+ parent <code>BillingDetails</code> instance.</para>
+ </callout>
+
+ <callout arearefs="inheritJoinSqlJava-4-co inheritJoinSqlJava-5-co"
+ xml:id="inheritJoinSqlJava-4">
+ <para>These constraints likewise describe <coref
+ linkend="inheritJoinSqlJava-2"/> and <coref
+ linkend="inheritJoinSqlJava-3"/> for
+ <code>BankAccount</code> datasets.</para>
+ </callout>
+ </calloutlist>
+
+ <para>Of course the NOT NULL constraints implement their
+ counterpart properties in the corresponding <trademark
+ xlink:href="http://www.oracle.com/us/technologies/java">Java</trademark>
+ objects.</para>
+
+ <para>The mapping does not cover one important integrity
+ constraint of our domain model. The base class
+ <classname>inherit.joined.v1.BillingDetails</classname> is
+ abstract. Thus each entry in the database must refer
+ either to a
+ <classname>inherit.joined.v1.CreditCard</classname> or a
+ <classname>inherit.joined.v1.BankAccount</classname>
+ instance. But the above database schema allows for
+ datasets in <code>BillingDetails</code> not being
+ referenced at all.</para>
+
+ <para>So the current database schema actually refers to a
+ domain model having a concrete base class
+ <code>BillingDetails</code>.</para>
+ </answer>
</qandaentry>
</qandadiv>
</qandaset>